Relative humidity (`relhum`) is a diagnostic variable.
It should not be included in MPAS input stream.

More elaborate error checking.
Also use a more descriptive name for number of constituents.

* Finish MPAS dynamical core initialization.
* Support for deferring the definition of constituents until run-time.
* Accessor functions/subroutines for querying:
  * Local mesh dimensions.
  * Constituent names and indexes.
  * Mapping between constituent indexes and MPAS scalar indexes.

It is now possible to construct an arbitrary list of variables
to input/output data to/from MPAS dynamical core. This functionality
may be overengineered, but it supports what CAM-SIMA needs.

There are two kinds of variables in MPAS: ordinary variables and
variable arrays. The old implementation would always skip the latter
entirely when encountering them during input.

The new implementation correctly distinguishes between the two, and
checks whether each variable is eligible to be input.

The instance of MPAS dynamical core resides in `dyn_comp`. `dyn_grid` depends on
`dyn_comp`, but not vice versa.

Maintain consistent style with others.

Same logic, but the statement is now more compact.

The statement is now clearer on its intent. Other similar instances in
the code already follow this style.

According to "8.4 Initialization", it states "Explicit initialization of a
variable that is not in a common block implies the SAVE attribute". This
also applies to pointer variables like here.

Avoid the implied SAVE attribute by removing explicit initialization. Uphold
the principle of least astonishment.

Easier to pinpoint where error occurs.

Zero or negative values are not allowed. Others are just fine.