Warn if files from the same file collection have different shapes

[tsalo]

I initially asked about this in https://github.com/bids-standard/bids-specification/issues/869. For some file collections (e.g., multi-echo or complex-values fMRI data), the files all come from the same acquisition and MUST have the same shape. For others (e.g., qMRI scans), there's no such restriction. As such, I think it would be very useful to raise a warning if the file collection contains files with different sizes.

[effigies]

Should have a think about how to do this in the schema. I think it would require a new association that all files in the collection share. Then we could say something like nifti_header.shape == associated.collection[0].shape.