Use pybids to manage data

[arokem]

Any BIDS-compliant directory should be a possible input, not only a directory organized the way that our AFQ.data module does it.

[arokem]

The challenge is that I don't think that pybids knows about derivatives yet...

[arokem]

This was implemented in #284, though there might be more work to do to use it more elegantly. At some point, we might want to have a look at niworkflows for inspiration. For now, I am closing this.