Modularize code for region selection and subsampling

[nikhilwoodruff]

          __suggestion:__ Modularize code for region selection and subsampling

This code feels a bit duplicative, and I'm wondering if these couldn't be functions, something like apply_subsample and apply_region. This would also prevent any possible future bug associated with modifying one piece of the code and accidentally forgetting the other.

This could also be a future thing, don't want to block where not necessary.

Originally posted by @anth-volk in #62 (comment)