ExtData2G backwards compatibility with non-CF vertical coord

[lizziel]

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)
• Tested this change with a run of GCHP

Description

This update is needed for ExtData2G compatibility with GCHP due to use of input files that are not CF-compliant. These files have vertical dimension variable with name 'lev', units 'level', and no positive attribute.

Changes are:

1. Categorize dimensions with name lev as vertical coordinate in ExtData2G (update provided by Atanas) as an alternative criteria to CF definition of vertical coordinate.
2. Assign positive attribute in ExtData2G for the case of vertical coordinate with no positive attribute in file and without pressure units. Do this by looking at standard_name, units, and order of dim values. Stop with error if standard_name is not level or levels, and units are not level or 1. Assumes positive:up if level values are increasing, and positive:down if decreasing.

Related Issue

None

[tclune]

@lizziel I made some stylistic suggestions in the review. I had meant to circle back and merge the suggestions, but GitHub is not letting me do that. Please click the buttons on your end, and I'll approve.

[lizziel]

Thanks @tclune for the suggestions!

[tclune]

My fault - but we need one more iteration on this now. Sorry about that.

[lizziel]

No worries. I should have done a more thorough review and tested it before committing.