Support file collections

[tsalo]

Summary

For the most part, file collections are composed of files from the same acquisition. The files typically have the same suffix, but vary along specific entities (echo, part, inv, flip, and mt). Most of the file collection-linking entities has associated metadata fields (e.g., EchoTime for echo, FlipAngle for flip).

In some cases (e.g., qMRI file collections), the files will come from different scans and may even have different fields of view 😬 (see bids-standard/bids-specification#869).

I'm struggling to come up with a plan for this.