Add utility to compare Fortran namelists

[aerorahul]

Description
Often times it is necessary to compare Fortran namelists between a UFS-weather-model regression test and a global-workflow experiment, or in other example applications.

This PR adds a simple utility that loads two namelists and spits out the differences between them. The differences are calculated as a departure from the first namelist.
This utility leverages f90nml (approved for use on WCOSS2)

The usage is as follows:

❯❯❯ python3 compare_f90nml.py -h
usage: compare_f90nml.py [-h] [-r] left_namelist right_namelist

Compare two Fortran namelists and display differences (left_namelist - right_namelist)

positional arguments:
  left_namelist   Left namelist to compare
  right_namelist  Right namelist to compare

options:
  -h, --help      show this help message and exit
  -r, --reverse   reverse diff (right_namelist - left_namelist) (default: False)

The comparison is done as follows:

• Both namelists are loaded
• We loop over the keys of left_namelist. We look for the same key in the right_namelist. If the key is found, the values are compared. If the key is not found, a note is made that the key is undefined in right_namelist.
• Differences in the values are printed to screen.

The -r | --reverse reverses the namelists. This allows the user to use right_namelist as the reference.

If differences are found, they are shown as follows (examples of input.nml from the control_p8 and cpld_control_p8 regression tests of the ufs-weather-model)

❯❯❯ python3 compare_f90nml.py control_p8.nml cpld_control_p8.nml
comparing: control_p8.nml | cpld_control_p8.nml
-----------------------------------------------
atmos_model_nml:
  ccpp_suite : FV3_GFS_v17_p8 | FV3_GFS_v17_coupled_p8
fms_nml:
  domains_stack_size : 3000000 | 8000000
fv_core_nml:
  dnats : 0 | 2
gfs_physics_nml:
    min_seaice : 0.15 | 1e-06
  use_cice_alb : False | True
     nstf_name : [2, 1, 0, 0, 0] | [2, 0, 0, 0, 0]
        cplchm : False | True
        cplflx : False | True
        cplice : False | True
        cplwav : False | True
    cplwav2atm : False | True

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?
As shown above in comparing 2 namelists from ufs-weather-model regression tests.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing tests pass with my changes

[WalterKolczynski-NOAA]

A few things, plus docstrings need arguments/returns (I like the descriptions).