Feature request: signal extraction in subject level space rather than group MNI template

[htwangtw]

This is an request from the cneuromod team.

Currently we are using the MNI template to access overlap of grey matter with atlas, but it would be useful to extend this to subject native space

[htwangtw]

At niprep sprint we learned about the nitransform: https://demo.hedgedoc.org/s/_OvFODeP8#
This should be a simple use case covered by nitransform.
For the load confounds part we might need to access the lower level api for this part