Speed up landice gridded_flood_fill with scipy.ndimage

[trhille]

Speed up gridded_flood_fill with scipy.ndimage instead of nested loops. For the antarctica_8km_2024_01_29.nc, this reduced the time from about 7s to 0.005s. This could make it much more manageable to use high-resolution gridded data sets to determine cell spacing for large meshes.

Checklist

• User's Guide has been updated
• Developer's Guide has been updated
• API documentation in the Developer's Guide (api.rst) has any new or modified class, method and/or functions listed
• Documentation has been built locally and changes look as expected
• The E3SM-Project submodule has been updated with relevant E3SM changes
• The MALI-Dev submodule has been updated with relevant MALI changes
• Document (in a comment titled Testing in this PR) any testing that was used to verify the changes
• New tests have been added to a test suite

[trhille]

Testing

When using the antarctica_1km_2024_01_29.nc source file, the current implementation of landice.mesh.gridded_flood_fill in main takes 495.47 seconds. With these changes, it takes 0.32 seconds.
@xylar @matthewhoffman, I think you'll like this :)

I still need to confirm that these result in similar masks.