Route model_mod calls for DART programs though assim_model_mod

[hkershaw-brown]

Description:

This pull request routes model_mod calls for DART programs though assim_model_mod.
model_mod_check was partially using assim_model_mod, and partially directly calling model_mod which
did not allow you to create a custom intercept layer using assim_model_mod (e.g having multiple model_mods).

Changes: call assim_model_mod rather than model_mod directly for dart programs

• Public model_mod routines through assim_mod_mod
• model_mod_check use assim_mod_mod::interpolate

FIXMES fixed:

• nc_write_model_atts was not going though assim_model_mod
• read_model_time in direct_netcdf_mod was not going though assim_model_mod

Left alone model specific programs, i.e. those in models directory and the two obs converters that use model info:

• create_identity_stream_flow_obs (models/wrf_hydro)
• convert_roms_obs (obs_converters/ROMS)

This is because the model specific code is making assumptions about the state, e.g. the identity obs, state indexing.
And I think programs in the models/MODEL directory are assuming they are not using the intercept layer.

Fixes issue

fixes #1056

Types of changes

• Bug fix (non-breaking change which fixes an issue)
  I'm putting bug fix since there are FIXMEs about this
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Documentation changes needed?

• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes.
run_all_quickdbuild.sh

For searching for occurrences:

• ripgrep -i -e 'use\s*model_mod\s*,\s*only'
• Excluding models directory: ripgrep -i -e 'use\s*model_mod\s*,\s*only' --glob '!models/*'

Note obs_timejitter.f90 is one of the programs that is never routinely compiled #1068, I will add quickdbuild.sh to exercise these program builds.

Checklist for merging

• Updated changelog entry
• Documentation updated
• Update conf.py

Checklist for release

• Merge into main
• Create release from the main branch with appropriate tag
• Delete feature-branch

Testing Datasets

• Dataset needed for testing available upon request
• Dataset download instructions included
• No dataset needed