Fixes #1888. Set regrid_method to identity for native collections

[mathomp4]

Description

This PR makes it so that if you have a native history collection, the regrid_method is set to identity. Before it was mistakenly set to bilinear. See #1888

Example:

        float H(time, lev, nf, Ydim, Xdim) ;
                H:_FillValue = 1.e+15f ;
                H:add_offset = 0.f ;
                H:coordinates = "lons lats" ;
                H:fmissing_value = 1.e+15f ;
                H:grid_mapping = "cubed_sphere" ;
                H:long_name = "edge_heights" ;
                H:missing_value = 1.e+15f ;
                H:regrid_method = "identity" ;
                H:scale_factor = 1.f ;
                H:standard_name = "edge_heights" ;
                H:units = "m" ;
                H:valid_range = -1.e+15f, 1.e+15f ;
                H:vmax = 1.e+15f ;
                H:vmin = -1.e+15f ;

Also, note to @bena-nasa I also changed the names of a couple of routines to get around a name clash as I wanted to add a get_regrid_method to the abstractregridder.

Related Issue

Closes #1888

Motivation and Context

Makes our output correct!

How Has This Been Tested?

Ran with GEOSgcm. Zero-diff save for metadata change.

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 change)
• Trivial change (affects only documentation or cleanup)

Checklist:

• I have tested this change with a run of GEOSgcm (if non-trivial)
• I have added one of the required labels (0 diff, 0 diff trivial, 0 diff structural, non 0-diff)
• I have updated the CHANGELOG.md accordingly following the style of Keep a Changelog

[mathomp4]

Also, ping @lizziel

[lizziel]

Looks good!