Upgrade sourmash categorize to take LCA databases as well as SBT.

[ctb]

sourmash categorize should be able to take an LCA database as well as an SBT.

There are a lot of other things we could do upgrade categorize as well; could brainstorm a little about what it is doing, should be doing, and could be doing.

[ctb]

Two idle thoughts --

• we could move categorize to the --db and --query command line UX used in sourmash lca classify, to permit multiple db.
• this is an opportunity to add a generic sourmash.load function that would load any kind of database.

[ctb]

ref #978

[ctb]

whups, turns out that categorize ignores self, and cannot be used efficiently on LCA databases.

[ctb]

also, here we probably want to remove LoadSingleSignature in favor of new load_file_as_signatures function.

[ctb]

This might be fixed in #1406.