Slow Indexing of fMRIPrep Directory

[wasciutto]

Indexing of fMRIPrep directory with pybids is extremely slow at scale - with 100 subjects and 4 tasks each, the indexing is north of 30 mins.

I tracked down the issue to be with a bug in pybids that is ignoring the flag telling pybids to not index metadata - I know what to do for the work-around, will be included in 1.8.2

[wasciutto]

closed but would still like to figure out how to exclude folders aside from func - continued in #401