transfer more SaveSignatures functionality over to the Index class?

[ctb]

I've shied away from implementing insert and/or save generically on Index classes, but it'd be useful to have (and, per #1936, have it be tested in a standard way). As it is in scripts we tend to use sourmash_args.SaveSignaturesToLocation(...) which has a fairly complete parallel implementation of signature saving logic. It'd be nice to switch this code over to just managing insertion and commit of the appropriate on-disk storage, with all the other logic moved into the Index class as possible.

this is a more general version of #1756...