Work with regional surface datasets

[samsrabin]

This PR makes it so that the FATES land-use tool is compatible with regrid_target_file being a surface dataset generated by the CTSM subset_data script. These are different from the usual surface datasets because:

• They already have lat and lon variables, although the associated dimensions are still lsmlat and lsmlon.
• In addition to the dimensions lsmlat and lsmlon, there are also coordinates with those names.

Needs to be tested for compatibility with the usual kind of surface dataset. But as far as regional surface datasets, it works for my South America domain, at least for the lupft file (cutoff at southern edge is intentional):

Remaining tasks

• Check that regional luh2 file looks okay.
• Check that runs forced with the regional outputs actually work.
• Check that outputs (luh2 and lupft) are unchanged when using normal surface datasets as regrid_target_file.

[samsrabin]

luh2 file also looks good.

[samsrabin]

This turned out to have been based on a very out-of date version of main. Replaced by #24.