rpn_signature_timeseries_.py creates unwanted folders in the derivatives folder

[khoffschlag]

The derivatives folder should only contain sub-xxx folders and a qc folder

[spisakt]

Atlas related sub-workflows are not subject specific but representative for the whole pipeline. We need to change the regexp in the decorator so that it puts all @grouppipeline derivatives into a single folder.

e.g. group/atlas-.nii.gz. # swap '_' to '-' to be more bids-like