make cmdline sigs optional for lca index

[bluegenes]

Makes sigs optional for lca index. Added a test with no cmdline signature, using --from-file. Arguments were already in correct order, thus tests did not need changing as in #1186.

Closes #1215.

• Is it mergeable?
• make test Did it pass the tests?
• 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.

(no new code)

• make coverage Is the new code covered?
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1222 into latest will increase coverage by 9.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1222      +/-   ##
==========================================
+ Coverage   84.22%   93.23%   +9.00%     
==========================================
  Files          99       75      -24     
  Lines        9232     5914    -3318     
==========================================
- Hits         7776     5514    -2262     
+ Misses       1456      400    -1056     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
sourmash/cli/lca/index.py	100.00% <ø> (ø)
src/core/tests/test.rs
src/core/src/sketch/minhash.rs
src/core/src/ffi/signature.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/sketch/ukhs.rs
src/core/src/from.rs
src/core/tests/minhash.rs
src/core/src/index/search.rs
src/core/src/signature.rs
... and 15 more

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[ctb]

looks ready for review, yah?

[bluegenes]

yep!

[ctb]

nice work!