[RFC] Enum for simplifying molecule checks

[luizirber]

Initial exploration on using enums for controlling how minhashes are processed (discussion in #751)

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 #765 into master will decrease coverage by 0.43%.
The diff coverage is 63.79%.

@@            Coverage Diff             @@
##           master     #765      +/-   ##
==========================================
- Coverage   88.42%   87.99%   -0.44%     
==========================================
  Files          30       31       +1     
  Lines        4786     4813      +27     
  Branches       46       47       +1     
==========================================
+ Hits         4232     4235       +3     
- Misses        551      576      +25     
+ Partials        3        2       -1

Impacted Files	Coverage Δ
sourmash/commands.py	88.15% <ø> (-0.15%)	⬇️
sourmash/kmer_min_hash.hh	99.04% <100%> (+0.98%)	⬆️
sourmash/hash_functions.py	22.22% <22.22%> (ø)
sourmash/signature_json.py	90.62% <52%> (-6.57%)	⬇️
sourmash/sourmash_args.py	96.18% <84.61%> (-0.4%)	⬇️
sourmash/lca/lca_utils.py	95.21% <0%> (-0.8%)	⬇️
sourmash/lca/command_index.py	90.26% <0%> (-0.45%)	⬇️
sourmash/sbt.py	85.21% <0%> (-0.32%)	⬇️
... and 2 more

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