Add ability to ingress QSIRecon derivatives for additional postprocessing

[tsalo]

Summary

The main derivative that we would want to postprocess is scalar maps, which could be mapped to bundles.

[arokem]

This would be great to have. In pyAFQ, we have the notion of inputs coming from another pipeline, and using bids filters to find these inputs in another derivative pipeline within the same BIDS dataset. The other pipeline could be a previous run of qsirecon, or something else. I think that would address this use-case and others. For example, we are interested in using an existing whole-brain tractography to create connectivity matrices with qsirecon.