Add FATES xarray functions repo as a submodule in tools

[glemieux]

Description:

This migrates the FATES-specific functions from the NCAR ctsm_python_gallery to it's own repository and makes the functions available as a submodule in tools.

Collaborators:

@samsrabin @ckoven

Expectation of Answer Changes:

None as this does not impact model code

Checklist

If this is your first time contributing, please read the CONTRIBUTING document.

All checklist items must be checked to enable merging this pull request:

Contributor

• The in-code documentation has been updated with descriptive comments
• The documentation has been assessed to determine if updates are necessary

Integrator

• FATES PASS/FAIL regression tests were run
• Evaluation of test results for answer changes was performed and results provided

Documentation

• Technical Note update:
• User's Guide update: Add fates xarray function usage documentation to the fates user's guide fates-users-guide#87

Test Results:

Not Applicable