coord_on_land: fix buffer for global grids

[tovogt]

Changes proposed in this PR:

• In u_coord.coord_on_land, use the more stable latlon_bounds function to add a buffer around the specified coordinates to avoid problems with global grids where the buffer would increase the bounds beyond 360 degrees in width.

This PR fixes issue #539

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[chahank]

Great, thanks for the fix!

@emanuel-schmid Should this maybe go directly into main as it is a bug?

[emanuel-schmid]

I'll cherry pick it into main.
er - maybe not. deviation from main is vast and cherry-picking doesn't work as expected.
Merging would too likely lead to other bugs. I leave it as it is and we'll fix the bug in main with the next release.