Generalize topo/combine to work on lat-lon grids

[xylar]

This PR generalizes the topo/combine workflow in e3sm/init to support lat/lon target grids, not just the cubed-sphere grid previously supported. This workflow combines the Bedmap3 topography data around Antarctica with the GEBCO data over the rest of the globe.

The PR adds support for 3 resolutions: 1, 0.25 and 0.0625 degrees. The first will just be used for very coarse, non-scientific meshes (if at all). The second might be used for low-res meshes and will be used for extrapolating the WOA 2023 dataset into ice-shelf cavities and land (WOA23 is on a 0.25 degree grid). The finest of these will be used as part of defining resolution for high-res unified MPAS meshes.

This PR also adds support for visualizing the topography on the lat-lon grid.

Checklist

• Developer's Guide has been updated
• API documentation in the Developer's Guide (api.md) has any new or modified class, method and/or functions listed
• Documentation has been built locally and changes look as expected
• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran the three new test cases on Perlmutter:

   2: e3sm/init/topo/combine_bedmap3_gebco2023/lat_lon/0.0625_degree/task
   3: e3sm/init/topo/combine_bedmap3_gebco2023/lat_lon/0.2500_degree/task
   4: e3sm/init/topo/combine_bedmap3_gebco2023/lat_lon/1.0000_degree/task

Here are a couple example plots at each resolution:

[xylar]

@katsmith133, this is the very first step in the Global Ocean workflow. If you don't mind giving the code an my plots a quick look, that would likely be sufficient. I believe I have also made the test results readable by you at:

/pscratch/sd/x/xylar/polaris_1.0/pm-cpu/test_20260411/topo-combine-lat-lon/e3sm/init/topo/combine_bedmap3_gebco2023/lat_lon

but Perlmutter is tricky with permissions. Let me know.

[xylar]

Updated docs are here:
https://portal.nersc.gov/cfs/e3sm/xylar/docs_testing/html/developers_guide/e3sm/init/tasks/topo/combine.html