Add max_threads_per_process and mp_context to pca by channel computation and PCA metrics
#1296
| Job | Run time |
|---|---|
| 18m 57s | |
| 13m 39s | |
| 7m 57s | |
| 12m 24s | |
| 15m 18s | |
| 9m 33s | |
| 1h 17m 48s |