change lca subcommands to use tax taxonomy code/handling

[ctb]

The CSV parsing code in src/sourmash/lca/command_index.py is just terribly complicated and parts of it (--no-headers, for example) are not even tested.

Given the ongoing maintenance and expansion of tax taxonomy handling code, we should switch to using it (mildly ironic, since some of the tax code was based on the LCA code, I think?)

ref simplifying the way we handle identifiers, #1962

[ctb]

this would also support gzip input... and maybe we can change it up to support 'default' taxonomy names... ref #2195 (comment) and #2012

[ctb]

this would also support gzip input... and maybe we can change it up to support 'default' taxonomy names...

now in #2216