Work with regional surface datasets (v2)

[samsrabin]

Supersedes #22, which was based on a very out-of-date version of main.

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 (cutoff at southern edge is intentional):

luh2 file:

lupft file :

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.