Update development SLIM to CESM2.3

[ekluzek]

SLIM is currently configured to work with the CESM2.1 release series. It's fairly easy to update to versions along the CESM2.1 series, and we have done that.

To have a development version of SLIM that is kept up to date with the latest CESM, we need to update to CESM2.3. Part of that means creating a new NUOPC cap for SLIM and doing the cime_config and coupler cap updates that are needed. I think the source code can largely remain the same, other than the development of the new NUOPC cap. There are some changes in cime_config that you can see in CTSM from CESM2.1 to CESM2.3 that will need to be moved over as well. But, largely the new python version of buildnml for SLIM should be good to be used in CESM2.3.

I'm creating this issue to track some of the things that will need to be done for that update, as I figure them out.

[ekluzek]

Updates include making python scripts use python3 rather than python

Changes in cime_config:

buildlib -- New subroutine _write_ctsm_mk
buildnml -- Add new driver option
will need to read in mesh file

[ekluzek]

For src/cpl
Move current files to "src/cpl/mct", add new directories for: src/cpl/nuopc, src/cpl/share_esmf, src/cpl/utils

Use of shr_log_errMsg in shr_log is new.
lnd_set_decomp_and_domain_from_surfrd in lnd_set_decomp_and_domain is new
seq_timemgr_EClockGetData call modified
get_curr_day calls has new reuse_day_365_for_day_366 argument
lnd_handle_resume is updated

Some changes to import/export and additional checking