Document and streamline/refactor signature loading and saving

[ctb]

I think this shows up in some other issues too, especially the loading, but from slack:

@taylorreiter

I’ve tried about 1 million and 1 combinations of things, and even read the code in unit tests, and can’t figure out how to write out a sourmash sig in the python api.

@olgabot:

Maybe worth writing some wrappers that take a list of signatures and a filename to save? Or a list of paths and returns all signatures?

[taylorreiter]

I think just adding:

with open(out_sig, 'wt') as fp:

so it reads:

with open(out_sig, 'wt') as fp:
      signature.save_signatures([sig], fp)

would be really helpful!

[ctb]

OK!

I found this -- and this. I think the latter is probably what are referring to above, in terms of adding with ....

[ctb]

See #921.

[ctb]

ref #875

[ctb]

also see #934.

[ctb]

per #875 (comment),

I think sourmash_args.load_database and sourmash_args.load_file_as_signatures should become the recommended public API for loading signatures; see #1059