Use RGD's `Fileset` for `DatasetRun` output instead of ManyToManyField

[mvandenburgh]

Instead of having output files of a dataset run as a ManyToManyField between DatasetRun and ChecksumFile, we can leverage RGD's existing file grouping mechanism and simply have a ForeignKey on the DatasetRun that points to a Fileset which contains all relevant ChecksumFiles.

@AlmightyYakob does this make sense to do?

[jjnesbitt]

I think this makes sense. While officially the semantics of that model in RGD is for grouping files which are intended to be ingested as one file, I think using it for this purpose here is fine 👍