revamp load_taxonomy_assignments and command_index.py

[ctb]

charcoal is making heavy use of load_taxonomy_assignments from lca/command_index.py, and there are a few things that are becoming annoying -- TODO items:

• add suppression of stdout/stderr output
• update to allow for filename-based selection of taxonomy

[ctb]

This was added in #1543 and #1651, sourmash.tax_utils.MultiLineageDB.load(...).