Fixes #2845. Allow ESMF_CONFIG_FILE to specify ESMF control file

[mathomp4]

Types of change(s)

• 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 change)
• Trivial change (affects only documentation or cleanup)
• Refactor (no functional changes, no api changes)

Checklist

• Tested this change with a run of GEOSgcm
• Ran the Unit Tests (make tests)

Description

This PR adds the ability for users to set an environment variable, ESMF_CONFIG_FILE, that can point to a file where pre ESMF_Initialize values can be set. Previously, the file had to be called ESMF.rc. Now, it looks at ESMF_CONFIG_FILE and if that isn't set, it then looks for ESMF.rc (for backwards compatibility).

I have tested this with GEOS and it works as expected.

@tclune if you have time, let me know if you have preferences for what to call the environment variable, etc.

Related Issue

Closes #2845