Fix lca gather to work on non-DNA databases.

[ctb]

In #1013, we added non-DNA LCA databases but failed to check that lca gather worked on them. This is now fixed. Also adds tests for loading LCA databases with incompatible moltypes, which should fail. And adds a deprecation warning for sourmash lca gather, which will be removed in future versions.

Fixes #1028
Fixes #728

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• 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]

Ready for review @bluegenes !

[codecov]

Codecov Report

Merging #1031 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1031      +/-   ##
==========================================
+ Coverage   92.31%   92.34%   +0.02%     
==========================================
  Files          72       72              
  Lines        5413     5421       +8     
==========================================
+ Hits         4997     5006       +9     
+ Misses        416      415       -1     

Impacted Files	Coverage Δ
sourmash/sbt.py	87.60% <ø> (-0.02%)	⬇️
sourmash/lca/command_gather.py	84.90% <100.00%> (+1.03%)	⬆️
sourmash/lca/lca_db.py	94.70% <100.00%> (+0.08%)	⬆️

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[ctb]

@luizirber let me know if you think we should do deprecations differently.

[bluegenes]

tests passing, spot check worked ... lgtm!