Scalar names that are more BIDS compliant

[arokem]

I just learned from @mattcieslak about their efforts to push ahead qsirecon (and the upcoming split between qsiprep and qsirecon). One of the elements of this is a push to be more compliant with the current BIDS BEP16 draft. Here is an example of what that looks like in practice:

https://github.com/PennLINC/qsirecon/blob/main/qsirecon/tests/data/scalar_mapper_outputs.txt

I think that we want to push in a similar direction with the filenames that we produce for model-derived scalars, but at the very least, we need to reconsider how the ImageFile object reads in scalars produced by other pipelines (i.e., this) so that we can easily ingest qsirecon outputs as part of our pipeline.