Various subset_data and related docs needed

[samsrabin]

From @ekluzek:

We need new instructions for the new methods involving subset_data. There are some hints in the PTCLM chapter about topics that should be put into the new subset_data section. In terms of things like how do you do this for US-UMB? As well as for some generic tower site you have data for? Instructions for making NEON and PLUMBER2 datasets should also be in the User's Guide. And instructions and examples on the modify surfdata tools would also be good to have as well.

For future reference, the "PTCLM chapter" referred to there can be restored by checking out a version of CTSM before the merge of whatever PR ends up resolving #2769. E.g., ctsm5.2.028.

Originally posted by @samsrabin in #1565