[WIP] add lca convert command

[ctb]

Fixes #1961.

May update to tackle #1959 and/or #1964, too.

[codecov]

Codecov Report

Merging #2000 (92f2fcd) into add/sqlite_index (9e616dd) will increase coverage by 7.35%.
The diff coverage is 61.42%.

@@                 Coverage Diff                  @@
##           add/sqlite_index    #2000      +/-   ##
====================================================
+ Coverage             83.99%   91.35%   +7.35%     
====================================================
  Files                   129      100      -29     
  Lines                 14937    10714    -4223     
  Branches               2079     2087       +8     
====================================================
- Hits                  12547     9788    -2759     
+ Misses                 2095      629    -1466     
- Partials                295      297       +2     

Flag	Coverage Δ
python	91.35% <61.42%> (-0.21%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/lca/command_convert.py	35.48% <35.48%> (ø)
src/sourmash/index/sqlite_index.py	90.92% <44.44%> (-0.81%)	⬇️
src/sourmash/cli/lca/convert.py	85.71% <85.71%> (ø)
src/sourmash/cli/lca/__init__.py	100.00% <100.00%> (ø)
src/sourmash/lca/__init__.py	100.00% <100.00%> (ø)
src/sourmash/lca/command_index.py	89.60% <100.00%> (-0.30%)	⬇️
src/sourmash/lca/lca_db.py	93.43% <100.00%> (+0.15%)	⬆️
src/core/src/ffi/storage.rs
src/core/src/index/search.rs
src/core/src/sketch/nodegraph.rs
... and 28 more

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[mr-eyes]

This looks good to me!

[mr-eyes]

I haven't noticed tests for lca_convert, though.