Added cubed_sphere target grid test to utility/combine_topo

[bmooremaley]

The ocean/utility/combine_topo test has been split into two separate tests: one for a lat_lon target grid and one for a cubed_sphere target grid. The cubed_sphere test allows the migration to mbtempest for remapping bathymetry to MPAS grids by removing the pole singularities. The workflow for the existing lat_lon target grid has also been updated to use remapping tools more consistently between the GEBCO and BedMachineAntarctica data sets, and between the lat_lon and cubed_sphere target mesh workflows. Specifically, the GEBCO dataset is separated into tiles prior to remapping, ESMF_RegridWeightGen is used for building weights files, and ncremap is used for the final remapping step.

Checklist

• User's Guide has been updated
• Developer's Guide has been updated
• Documentation has been built locally and changes look as expected
• Document (in a comment titled Testing in this PR) any testing that was used to verify the changes

[xylar]

@bmooremaley, this is excellent work! I'm very happy with the way you have reorganized things and the clean-up you've done along the way.

I have some suggested changes that we can discuss today.

[xylar]

Presumably, you'll need to do a commit to make the linter happy. The easiest way to do that is probably to add an extra newline somewhere (maybe at the end of the file) and then fix whatever it complains about. Make sure you do that with the conda environment loaded so pre-commit can happen.