make LCA signatures() method return signatures with the correct original name

[luizirber]

In 2020-cami I built an LCA index out of an SBT index (relevant snakemake rules). While writing a script for extracting accessions from an index (in https://github.com/dib-lab/2019-12-12-sourmash_viz) I noticed that the .signatures method for LCA is not returning the original name from the signature, but the md5sum.

So, how does gather return the correct name? 🤔

Turns out that the logic for picking the right name is in find_signatures, so I just moved it to _signatures instead (and made _signatures store Signatures instead of MinHashs). Now signatures from the .signatures method have the original signature name.

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #917 into master will increase coverage by 0.28%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #917      +/-   ##
==========================================
+ Coverage   91.30%   91.59%   +0.28%     
==========================================
  Files          71       71              
  Lines        5048     5054       +6     
==========================================
+ Hits         4609     4629      +20     
+ Misses        439      425      -14     

Impacted Files	Coverage Δ
sourmash/lca/lca_db.py	93.87% <100.00%> (+0.10%)	⬆️
sourmash/sbt_storage.py	95.50% <0.00%> (-1.09%)	⬇️
sourmash/sbt.py	85.69% <0.00%> (+0.28%)	⬆️
sourmash/lca/command_index.py	89.94% <0.00%> (+0.55%)	⬆️
sourmash/lca/lca_utils.py	96.55% <0.00%> (+2.29%)	⬆️
sourmash/_compat.py	100.00% <0.00%> (+50.00%)	⬆️

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[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
1 Code Smell

No Coverage information
0.0% Duplication

[ctb]

logic seems good. test?

[luizirber]

@ctb I think this was covered in #946 ?

[ctb]

Alas, it was not! Fixed. Review & merge when ready.