Remove loops over calendar types where not needed

[mjs2369]

Description:

The subroutines set_calendar_type_string and get_calendar_string in the time_manager_mod have unnecessary loops over the number of max_types (calendar types). This PR removes these loops for improved code efficiency and readability

Fixes issue

fixes #653

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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

Ran model_mod_check and filter with print statements to ensure the correct calendar_type was being set/used

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

[hkershaw-brown]

For commits like these, where you've just missed something from the commit (removing the loop variable) and there aren't a bunch of people working with the pull request, you can rebase and squash the commits together.
I'll go ahead an squish them.

[hkershaw-brown]

Looks good Marlee!

For reference I added (and tested with) a unit test in the DART-tests repo:
https://github.com/NCAR/DART-tests/tree/main/unit/time_manager_mod