CTSM init. conditions in WRF-CTSM: translate ncl script to python

In this week's WRF-CTSM meeting (2020/11/17 - attended by @dlawrenncar , Fei Chen, @billsacks @negin513 @slevisconsulting ) we discussed CTSM initial condition issues in the WRF-CTSM context. We tentatively agreed the following:
- No change in method at this time, i.e. we continue to use CTSM's finidat file and import spun up data from WRF initial conditions if desired. Importing spun up data currently happens with an ncl script written by @barlage and modified for a special case by @slevisconsulting .
- We need to translate the ncl script to python and that's the motivation for opening this issue.

The ncl script is used as follows:
`ncl transfer_wrfinput_to_ctsm_with_snow.ncl 'finidat="the_existing_finidat_file.nc"' 'wrfinput="your_wrfinput_file"' 'merged="the_merged_finidat_file.nc"'`

@slevisconsulting has three copies of the ncl script:
`/glade/work/slevis/git_wrf/ctsm_init/transfer_wrfinput_to_ctsm_no_snow.ncl`
`/glade/work/slevis/git_wrf/ctsm_init/transfer_wrfinput_to_ctsm_nwp_fast_w_snow.ncl`
`/glade/work/slevis/git_wrf/ctsm_init/transfer_wrfinput_to_clm_std_w_snow.ncl`