investigate taking stdin in sourmash sig submodule and/or load_signatures

[ctb]

For reasons that I haven't chased down, commands like

cat tests/test-data/47.fa.sig | sourmash sig describe -

don't work. I'm not sure there's a real reason for this - it should work - so we should fix it!

It'd be nice if sig cat, sig split, etc. could take stdin in addition to filenames. A unified loading framework as suggested in #875 could also help with this.