Add the ability to set the threads for CPU usage.

[OWissett]

Currently, surface ID is very greedy with it threads, and will just take all of the threads it can. This is causing some issues for my team as we have a shared compute server. If we were running this via SLURM, this would not be an issue, however, we are typically running via notebooks, which makes this process a bit more tedious.

I am going to create a PR which implements a SURFACE_ID_MAX_THREADS env var which can be set to override the greedy nature of this code.