Play friendly with datalad datasets at the input

[oesteban]

If:

• bids root looks like a datalad dataset
• datalad is installed
• the user sets some --datalad-get flag (off by default)

A datalad get call is issued.

This could be implemented at the level of accessing the file or in the beginning. Both have pros and cons:

• At the beginning: fMRIPrep would execute datalad.get() on the NIfTI files selected by the BIDSLayout query. This greedy solution would be the easiest, but would perhaps take a long time fetching a lot of files that might not be used after all.
• When data is accessed first: set up some interface that does this when the workflow is actually running, getting the files as they are needed.