Replaces the old gfs_cyc method for determining cycling frequency with one that
explicitly sets the step length. In place of gfs_cyc there are two new variables
to determine step sizing: STEP_GFS and STEP_DA. Both are of the timedelta type.

STEP_GFS determines the frequency of GFS and defaults to 6 hours
STEP_DA determines the frequency of DA and also defaults to 6 hours

If STEP_GFS is set to 00:00:00, only the DA system is run
If STEP_DA is set to 00:00:00, only the GFS system is run (free-forecast mode)

Note that this new method currently requires the additional setting of STEP_DA
to zero, in addition to using free forecast layout.

If STEP_DA is non-zero, the first cycle is DA-only, so this should be kept in
mind when setting up your experiments (especially if you want GFS to run at
certain times of day). If both are non-zero, STEP_GFS should always be an
integer multiple of STEP_DA, otherwise unexpected behavior may occur. Other
than setting STEP_DA to zero for free forecast, STEP_DA is unlikely to be
changed from its default value in the near-future for GFS, but the new method
is flexible for any future incorporation of LAM system that requires faster DA
cycling.

Examples:

Full-cycling with DA & GFS:
STEP_GFS: !timedelta "06:00:00"
STEP_DA: !timedelta "06:00:00"
(since these are defaults, nothing needs to be placed in the case file, but you may
wish to for clarity)

Normal DA with GFS at each 00z starting 2013-04-01:
SDATE: 2013-03-31t18:00:00
STEP_GFS: !timedelta "24:00:00"

Free forecast with GFS every 24h at 00z starting 2013-04-01:
SDATE: 2013-04-01t00:00:00
STEP_GFS: !timedelta "24:00:00"
STEP_DA: !timedelta "00:00:00"

CROW ss updated to add a new tool that converts a timedelta to a string.

The wave model, which formerly used gfs_cyc to determine the wave cycling interval,
now determines WAVHCYC directly from STEP_GFS. Since this variable is only a number
of hours, this will only work for whole-hour GFS frequencies.

Case files have been updated to use the new settings in place of gfs_cyc, as well as
set STEP_DA to zero for free forecast cases. However, they have not all been tested.

Additional testing is needed to ensure these changes are fully operational for DA
cycling runs.

Refs: NOAA-EMC#260, NOAA-EMC#314

Replaces the old gfs_cyc method for determining cycling frequency with one that
explicitly sets the step length. In place of gfs_cyc there are two new variables
to determine step sizing: STEP_GFS and STEP_DA. Both are of the timedelta type.

STEP_GFS determines the frequency of GFS and defaults to 6 hours
STEP_DA determines the frequency of DA and also defaults to 6 hours

If STEP_GFS is set to 00:00:00, only the DA system is run
If STEP_DA is set to 00:00:00, only the GFS system is run (free-forecast mode)

Note that this new method currently requires the additional setting of STEP_DA
to zero, in addition to using free forecast layout.

If STEP_DA is non-zero, the first cycle is DA-only, so this should be kept in
mind when setting up your experiments (especially if you want GFS to run at
certain times of day). If both are non-zero, STEP_GFS should always be an
integer multiple of STEP_DA, otherwise unexpected behavior may occur. Other
than setting STEP_DA to zero for free forecast, STEP_DA is unlikely to be
changed from its default value in the near-future for GFS, but the new method
is flexible for any future incorporation of LAM system that requires faster DA
cycling.

Examples:

Full-cycling with DA & GFS:
STEP_GFS: !timedelta "06:00:00"
STEP_DA: !timedelta "06:00:00"
(since these are defaults, nothing needs to be placed in the case file, but you may
wish to for clarity)

Normal DA with GFS at each 00z starting 2013-04-01:
SDATE: 2013-03-31t18:00:00
STEP_GFS: !timedelta "24:00:00"

Free forecast with GFS every 24h at 00z starting 2013-04-01:
SDATE: 2013-04-01t00:00:00
STEP_GFS: !timedelta "24:00:00"
STEP_DA: !timedelta "00:00:00"

CROW ss updated to add a new tool that converts a timedelta to a string.

The wave model, which formerly used gfs_cyc to determine the wave cycling interval,
now determines WAVHCYC directly from STEP_GFS. Since this variable is only a number
of hours, this will only work for whole-hour GFS frequencies.

Case files have been updated to use the new settings in place of gfs_cyc, as well as
set STEP_DA to zero for free forecast cases. However, they have not all been tested.

Additional testing is needed to ensure these changes are fully operational for DA
cycling runs.

Refs: NOAA-EMC#260, NOAA-EMC#314

To facilitate longer and more flexible GFS cadences, the `gfs_cyc`
variable is replaced with a specified interval. Up front, this is
reflected in a change in the arguments for setup_exp to:

```
--interval <n_hours>
```

Where `n_hours` is the interval (in hours) between gfs forecasts.
`n_hours` must be a multiple of 6. If 0, no gfs will be run (only
gdas; only valid for cycled mode). The default value is 6 (every
cycle).

In cycled mode, there is an additional argument to control which
cycle will be the first gfs cycle:

```
---sdate_gfs <YYYYMMDDHH>
```

The default if not provided is `--idate` + 6h (first full cycle).

As part of this change, some of the validation of the dates has
been added. `--edate` has also been made optional and defaults to
`--idate` if not provided.

During `config.base` template-filling, `INTERVAL_GFS` (renamed from
`STEP_GFS`) is defined as `--interval` and `SDATE_GFS as
`--sdate_gfs`.

Some changes were necessary to the gfs verification (metp) job, as
`gfs_cyc` was being used downstream by verif-global. That has been
removed, and instead workflow will be responsible for only running
metp on the correct cycles. This also removes "do nothing" metp
tasks that exit immediately, because only the last GFS cycle in a
day would actually process verification.

Now, metp has its own cycledef and always runs at 18z, regardless
of whether gfs is running at 18z or not. This is simplier than
trying to determine the last gfs cycle of a day when it could
change from day to day. To facilitate this change, support for the
undocumented rocoto dependency tag `taskvalid` is added, as the
metp task needs to know whether the cycle has a gfsarch task or not.
metp will trigger on gfsarch completing (as before), or gdasarch
completing if there is no gfsarch.

metp tasks are no longer generated for forecast-only, as the pgbanl
files (copied of the 1p00 pgbanl files) are not generated for f-o
anyway. If metp is needed for f-o, additional work will be needed.

Additionally, a couple EE2 issues with the metp job are resolved
(even though it is not run in ops):
- verif-global update replaced `$CDUMP` with `$RUN`
- `$DATAROOT` is no longer redefined in the metp job

Depends on NOAA-EMC/EMC_verif-global#137
Resolves NOAA-EMC#260
Refs NOAA-EMC#1299