BEP: Make proc filename keyword optional for other (T1, EPI, etc) modalities

[yarikoptic]

Continuing my trend (#47) of demands, I would like to add _proc to the campaign.
The original usecase (nipy/heudiconv#266) is NORMalized (intensity normalization performend by the scanner software) T1w and T2w anatomicals. NORM'ed files accompany non-normalized original raw data. Sure thing I could just consider them "derivative" images and drop them, but some researchers might believe in that preprocessing superiority and demand them to be present in their raw BIDS datasets since that is what they get from the scanner.
Given that there is now _proc field (introduced optional for stim and meg) I do not see why it should not be allowed (optional as well) for pretty much any other modality (in particular anat, bold, dwi).

[chrisgorgo]

Why not use rec for this purpose - it's already in the spec?

[yarikoptic]

Because it is not about reconstruction type. Reconstruction was the same afaik, only to one postprocessing was applied. If I were to stick it to some random field I would probably use _acq, but that is the point - why to stick into a random field with loose or inappropriate semantic?

[chrisgorgo]

Please open a pull request with the change and we will see if anyone objects.