Copy custom templates to sourcedata directory

[tsalo]

Is your feature request related to a problem? Please describe.
It's not a problem, but it does seem like, once BEP038 is merged into BIDS, it would be appropriate to include custom templates stored in the TemplateFlow directory as BIDS-format templates in sourcedata/, much like the atlas-4S example in bids-standard/bids-examples#501.

Describe the solution you'd like
I think sMRIPrep could copy the relevant templates to sourcedata and organize them as a BIDS template dataset. Then SpatialReferences and such could reference that folder instead of the local TemplateFlow folder.

Describe alternatives you've considered
Keep things as-is.

Additional context