_mod entity: should it be kept, dissolved, transformed, or embraced?

[yarikoptic]

_mod was introduced and is used primarily to disambiguate different _defacemasks. So, to a degree, it encapsulates the semantic that some, but not all, _suffixes correspond to MRI modalities (really) as a summarized in a comment to #58.

• Remove the suffix and exclusively use entities in the filename. #58

It is only present only when there is a _defacemask and only for anat/, while in effect most of the time corresponding to those suffixes "implicitly" where suffix does correspond to "contrast" (not modality really).

Samples from openneuro

$> for ds in ds*; do f=$(find $ds -iname *_mod-* | head -n 1); [ -z "$f" ] && continue; echo "$f"; done    
ds000113/sub-01/ses-forrestgump/anat/sub-01_ses-forrestgump_acq-mag_mod-veno_defacemask.nii.gz
ds000224/sub-MSC01/ses-struct01/anat/sub-MSC01_ses-struct01_run-01_mod-T1w_defacemask.nii.gz
ds000233/sub-rid000001/anat/sub-rid000001_mod-T1w_defacemask.nii.gz
ds001297/sub-01/anat/sub-01_acq-filt_mod-T1w_defacemask.nii.gz
ds002718/sub-002/anat/sub-002_mod-T1w_defacemask.nii.gz
ds003645/sub-002/anat/sub-002_mod-T1w_defacemask.nii.gz
ds004192/sub-01/ses-localizer1/anat/sub-01_ses-localizer1_acq-prescannormalized_rec-pydeface_mod-T1w_defacemask.nii.gz
ds004712/sub-03286/ses-NOT1ACH001/anat/sub-03286_ses-NOT1ACH001_mod-T1w_defacemask.nii.gz

Also there is

• Change modality directories to entity format #55

where the point of having _mod is coming up. And as we are planing to proceed with

• Make it possible to specify folders layout to be other than sub-{label}/[ses-{label}/] #54

if we get an entity _mod which actually corresponds to modality (mri, eeg, ...) not just mere contrast, it somewhat makes it then explicit and consistent to then "embrace" that entity and use it consistently to disambiguate all files, not just _defacemasks.

Note: if to be kept just for the sake of disambiguation of some _suffix2 (e.g. _defacemask) among different _suffixes (e.g. _T1w and _T2w) as it is now, yet another use is _events.tsv files which came up in the context of

• [ENH] Remove explicitly worded requirement of events file association with the task bids-specification#1440

[Remi-Gau]

• relates to this old issue Is <source_entities> sufficient or should be extended to include _mod- when necessary? bids-specification#625