Index.select(...) currently allows incorrect moltypes

[ctb]

This should raise an error, but does not:

db = sourmash.load_file_as_index('tests/test-data/63.fa.sig')
db = db.select(moltype='FOO')

Currently, the only valid moltypes are 'DNA', 'protein', 'dayhoff', and 'hp'.

Your mission, should you choose to accept it, is to make sure that this is generally true.

I think the function to modify is src/sourmash/index.py:select_signature(...), and it should raise a ValueError for invalid moltypes. This function is used by all of the Index subclasses in index.py and so fixing it should be enough.

Please also add a test or three in tests/test_index.py.

The other major Index subclasses, SBT and LCA databases, should be fine, but maybe worth a quick check.

see also #1427 -> more tests needed for Index.select