[FEATURE REQUEST] Consolidate and rename regridding files and functions

[lizziel]

We currently have file_regrid.py and regrid_restart_file.py. The former can regrid restart files which makes this naming confusing. The key difference between the two is file_regrid.py uses regridding weights computed during runtime (online) while regrid_restart_file uses externally produced weights (offline). This is not very intuitive so we should clean it up to be more clear.

Tagging @yantosca and @kilicomu. This update would go on top of recent regridding work in #266.

[lizziel]

Included in this feature request is to also update the names passed to the regridding functions to be more clear about latlon versus cubed sphare, and type of cubed sphere format. Currently we give 3 options: classic, checkpoint, and diagnostic. We could change this to be latlon, cs_restart, and cs_diagnostic, or break up the argument into grid type and file format: latlon and cubedsphere for grid type, standard and mapl_restart for format. These are just some initial ideas. Open to other ideas too!

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