[MRG] fix gather against signatures with abundances

[ctb]

The refactoring of gather in #1370 introduced a bug where matching signatures were not flattened, which caused gather to exit with a TypeError exception. This PR provides a fix and a test.

Fixes #1518

@taylorreiter ready for review!

[codecov]

Codecov Report

Merging #1519 (336d336) into latest (1633121) will increase coverage by 5.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1519      +/-   ##
==========================================
+ Coverage   90.24%   95.27%   +5.03%     
==========================================
  Files         126       99      -27     
  Lines       21104    17409    -3695     
  Branches     1589     1589              
==========================================
- Hits        19046    16587    -2459     
+ Misses       1829      593    -1236     
  Partials      229      229              

Flag	Coverage Δ
python	95.27% <100.00%> (+<0.01%)	⬆️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/search.py	96.24% <100.00%> (ø)
tests/test_sourmash.py	99.72% <100.00%> (+<0.01%)	⬆️
src/core/tests/test.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/linear.rs
src/core/src/errors.rs
src/core/src/wasm.rs
src/core/src/signature.rs
src/core/src/ffi/mod.rs
src/core/src/index/sbt/mhbt.rs
... and 19 more

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[taylorreiter]

Woo! Thanks, LGTM. Can confirm, works on real signatures that have abundances.