[MRG] add --num-results/-n to gather

[ctb]

Provides -n/--num-results argument to sourmash gather.

Fixes #1042.

• 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]

This should be ready for review @erikyoung85 @bluegenes

[codecov]

Codecov Report

Merging #1047 into master will increase coverage by 9.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1047      +/-   ##
==========================================
+ Coverage   83.51%   92.55%   +9.04%     
==========================================
  Files          99       74      -25     
  Lines        8880     5590    -3290     
==========================================
- Hits         7416     5174    -2242     
+ Misses       1464      416    -1048     

Flag	Coverage Δ
#rusttests	?

Impacted Files	Coverage Δ
sourmash/cli/gather.py	100.00% <100.00%> (ø)
sourmash/commands.py	86.87% <100.00%> (+0.09%)	⬆️
src/core/src/ffi/mod.rs
src/core/src/from.rs
src/core/src/index/linear.rs
src/core/src/wasm.rs
src/core/src/index/sbt/mhbt.rs
src/core/tests/signature.rs
src/core/src/index/bigsi.rs
src/core/src/ffi/signature.rs
... and 17 more

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[ctb]

updated & ready for review again :)