Allow external atlases organized as BIDS datasets

[tsalo]

Summary

Once in a while, users will request that we add a new atlas to the list of built-in atlases for XCP-D. Unfortunately, that list is growing quite long at this point. I would prefer to have a solid way to let users provide their own atlases.

The relevant BEP, BEP038: Atlas Specification, proposes adding an "Atlas" dataset type. XCP-D currently writes out its atlases in a manner that somewhat follows the BEP, so I don't think it's a massive stretch to ingress atlas datasets to be used for parcellation.

Additional details

We may need to wait until the Atlas BEP is finalized and merged before XCP-D can officially start supporting it.

Next steps

1. Allow atlases in --atlases to be specified with the label=path structure, like how fMRIPrep handles --derivatives.
2. Load the relevant information from the atlas dataset.
3. In the near term, limit atlases to ones in MNI152NLin6Asym space (for nifti processing) or fsLR+MNI152NLin6Asym space (for cifti processing). In the long term, we should be able to use workflows from fMRIPost pipelines to warp to whatever space is necessary- at least for niftis.