[MRG] add selector framework to Index, and moltype property to MinHash object

[ctb]

When working on #809 I decided to try adding a function Index.select(ksize=ksize, moltype=moltype, ...) that would return a version of the Index that contained only signatures that matched those requirements. I also added a moltype property to the MinHash class.

These changes are more general than #809 and touch on a few other issues, like #599, and deserve to be tested and reviewed separately, so I'm breaking them out here.

• 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?

[ctb]

(FWIW I'm thinking of moving the moltype property to the MinHash object.)

[codecov]

Codecov Report

Merging #936 into master will decrease coverage by 0.31%.
The diff coverage is 54.05%.

@@            Coverage Diff             @@
##           master     #936      +/-   ##
==========================================
- Coverage   91.89%   91.58%   -0.32%     
==========================================
  Files          71       71              
  Lines        5021     5049      +28     
==========================================
+ Hits         4614     4624      +10     
- Misses        407      425      +18     

Impacted Files	Coverage Δ
sourmash/sbt.py	85.69% <10.00%> (-1.10%)	⬇️
sourmash/lca/lca_db.py	93.77% <11.11%> (-2.66%)	⬇️
sourmash/_minhash.py	94.60% <100.00%> (-0.42%)	⬇️
sourmash/index.py	98.59% <100.00%> (+0.20%)	⬆️

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities (and 0 Security Hotspots to review)
2 Code Smells

No Coverage information
0.0% Duplication

[ctb]

I think this is ready for a first round of comments, @luizirber, if you are interested :)

[luizirber]

yes. See #751 for longer discussion, especially #751 (comment)

[luizirber]

Do you want to go all the way here and implement moltype/hash_function as an enum? Like a918b26 did (equivalent to #765 in Rust, implementing ideas from #751?)

[ctb]

(remaining comments moved to new issues)