Ability for user to specify automatic vs. number of cores

[ColinOrionChandler]

There are places in the region_search and other code where the number of processes for multiprocessing is specified.

As I'm running stuff on Klone, I'd love it if we can set to "automatic" (read: None), which triggers the max_cpus()+2 or w/e the default is for a given approach (e.g., tqdm process_map).