[MRG] fix two bugs in gather --output-unassigned

[ctb]

(Stealing #1151 from @olgabot, sorry olga :))

Inspired by #1151, I dug into the --output-unassigned code in sourmash gather and found not just the moltype bug in #1151 but also a bug where even fully gather'd inputs would result in saving an (empty) signature file. This PR:

• adds a test for protein signature output via --output-unassigned, fixes Output unassigned hashes from "gather" to same moltype #1151
• adds a check for no output from --output-unassigned when the query signature is completely allocated
• refactors the code in --output-unassigned to be much simpler 😂

• 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 @olgabot

[codecov]

Codecov Report

Merging #1156 into latest will increase coverage by 8.91%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1156      +/-   ##
==========================================
+ Coverage   83.72%   92.63%   +8.91%     
==========================================
  Files          95       71      -24     
  Lines        9012     5692    -3320     
==========================================
- Hits         7545     5273    -2272     
+ Misses       1467      419    -1048     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
sourmash/commands.py	87.01% <100.00%> (+0.03%)	⬆️
sourmash/nodegraph.py	77.27% <0.00%> (-16.37%)	⬇️
src/core/src/ffi/nodegraph.rs
src/core/src/sketch/ukhs.rs
src/core/src/ffi/minhash.rs
src/core/src/ffi/utils.rs
src/core/src/index/search.rs
src/core/src/lib.rs
src/core/tests/minhash.rs
src/core/src/index/bigsi.rs
... and 16 more

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[olgabot]

Looks great, thank you so much! The protein test was a little obtuse to me, so added an explicit line for testing the moltype in the future. I know it's technically already covered by the x.minhash == y.minhash but it made it easier for me to understand.

[ctb]

excellent! merging!