picklists cause bug in LCA databases when relevant signatures are excluded

[ctb]

as of 9dbd8b5, there's a bug (almost certainly caused by #1588) where if a relevant signature in an LCA database is excluded by a picklist (so it doesn't show up in self._signatures) then LCA_Database.find(...) fails on this line:

subj = self._signatures[idx]

because the idx is not present.

For example,

sourmash gather tests/test-data/47.fa.sig tests/test-data/lca/47+63.lca.json --picklist ident.csv:ident:ident

where ident.csv is

ident
NC_011663.1

yields

Starting prefetch sweep across databases.
Traceback (most recent call last):
  File "/Users/t/miniconda3/envs/py37/bin/sourmash", line 33, in <module>
    sys.exit(load_entry_point('sourmash', 'console_scripts', 'sourmash')())
  File "/Users/t/dev/sourmash/src/sourmash/__main__.py", line 13, in main
    return mainmethod(args)
  File "/Users/t/dev/sourmash/src/sourmash/cli/gather.py", line 89, in main
    return sourmash.commands.gather(args)
  File "/Users/t/dev/sourmash/src/sourmash/commands.py", line 687, in gather
    counter = db.counter_gather(prefetch_query, args.threshold_bp)
  File "/Users/t/dev/sourmash/src/sourmash/index.py", line 278, in counter_gather
    for result in self.prefetch(prefetch_query, threshold_bp, **kwargs):
  File "/Users/t/dev/sourmash/src/sourmash/index.py", line 217, in prefetch
    for sr in self.find(search_fn, query, **kwargs):
  File "/Users/t/dev/sourmash/src/sourmash/lca/lca_db.py", line 473, in find
    subj = self._signatures[idx]
KeyError: 32