FIX: Traverse BIDS hierarchy to find masks, bvals, and bvecs

[richford]

This PR traverses the BIDS hierarchy to find masks, bval files, and bvec files instead of assuming that they will be found in the same session. It is part bugfix, part enhancement because it also fixes the underscore error in PFTMask.get_mask().

Traversing the hierarchy should allow it to interact with common output patterns from nipreps.

[pep8speaks]

Hello @richford! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2020-11-06 06:02:40 UTC

[richford]

@arokem and @36000, assuming the documentation builds pass, I think this is ready for review. In the latest commit, I changed all internal .bvals and .bvecs extensions to .bval and .bvec. As far as I know, the plural versions are not BIDS-compliant and they were making pybids mad.

[36000]

I changed some of the nighlty smoke tests to run tractography when testing tractography related things. If this runs into emmory errors I can reduce number of seeds, but that should be in a different PR. These new tests would have been able to trigger your error, I think.