BIDS dataset indexing

[kaitj]

Adding this here for discussion and so it isn't forgotten.

PyBIDS can get inefficient for large datasets. To that end, we came across bids2table (https://github.com/cmi-dair/bids2table) at OHBM. @tkkuehn and I both tried it out today on arbitrary, large datasets, and despite a few hiccups, it was fairly easy to use with substantial speed boosts.

May be early to replace PyBIDS usage in Snakebids, but wanted to open up the discussion for potentially replacing it down the line.