Missing B1- maps

[ChristophePhillips]

the fmap folder was originally planned to host the B0 fieldmap, mainly for fMRI/EPI data. With advanced anatomical images, 2 other types of fieldmaps are regularly acquired: B1+ and B1 maps.
B1+ maps, aka. "B1 bias correction". These have been added in the current version of the BEP001 proposal, with the following template.

sub-<participant_label>/[ses-<session_label>/]
    fmap/
        sub-<participant-label>[_ses-<session_label>][_acq-<acq-label>][_run-<run_index>]_B1plusmap.nii[.gz]
        sub-<participant-label>[_ses-<session_label>][_acq-<acq-label>][_run-<run_index>]_B1plusmap.json

These images are thus clearly designated by the B1plusmap suffix.

A similar suffix, e.g. B1minusmap, could be introduced in order to host the B1- maps in the same fmap folder. The template would be pretty similar, apart for the suffix of course.