At this point it's possible to associate file groups with related file groups.
This is handled symmetrically.
Attaching the note field is not yet supported, but hopefully won't
pose a large problem.
Add abillity to have note attached to related file group. I needed to
experiment a bit to even have an idea how to make it work,  so tests
are pending as of this commit.
Added test for notes on related file groups.
refactoringg
Add comment.
Extract partial (to remove clutter and to add comment).
The related file handling can be done in an around method.