ASL Context issues with 3D ASL NIfTIs

[aptinis]

1. According to the BIDS spec, *_asl.nii.gz may sometimes consist of only 1 volume if the scanner only produces a deltam or cbf map, such as for the GE product PCASL sequence. The number of volumes expected when validating the the ASL Context file should be 1 for 3D ASL NIfTIs. d2b-asl crashes because it is determining the number of volumes from the last element of the image shape. For 3D NIfTIs the last element of their shape is the number of slices not the number of volumes resulting in a mismatch in the expected number of lines in the ASL Context file.

2. According to the BIDS spec, if the m0 scan is separate (*_m0scan.nii.gz) it should not have an accompanying *_aslcontext.tsv.