adjust Python API for loading and saving signatures?

[ctb]

Some ideas rescued from #1142 -

• load_file_as_signatures doesn't take a string input, per [MRG] Switch use of load_signatures over to load_file_as_signatures throughout. #1304. This challenges it as a replacement for sourmash.signature.load_signatures... I wonder if we can let load_file_as_signatures take a fp, instead of a string as load_signatures does? Then we could support file objects and (via io.StringIO) string inputs.

• do we we want to change save_signatures to require a file output, OR alternatively provide a new top-level function that requires you write to a file?

[ctb]

side note, save_signatures should maybe be adjusted to require kwargs, something like

def save_signatures(sigs, fp=None, *, compression=0)

[ctb]

#1493 provides flexible signature saving functionality that supports Zip files, directories, and sig files.

We might want to add something to this class that makes it a single function call to write a bunch of signatures. Right now it requires multiple calls b/c of the primary use case, but this is easy to fix.

Other changes might include using ZipStorage underneath #1441.

[ctb]

(the "single function call to write a bunch of signatures" is a good next issue!)

[ctb]

ref #1877