[MRG] report both weighted and unweighted % recovered in gather

[ctb]

This PR changes sourmash gather to report both weighted and unweighted k-mer identification per #1818; the changes in #2249 made this a pretty easy fix, and the conversation in dib-lab/genome-grist#197 made it obvious that we should support it for genome-grist purposes!

This PR also updates the docs on % recovered in response to #2300 (comment).

example output

% sourmash gather SRR606249-abund-10k.sig.zip  podar-ref.zip 
...
found 64 matches total;
the recovered matches hit 94.0% of the abundance-weighted query.
the recovered matches hit 45.6% of the query k-mers (unweighted).

If --ignore-abundance is specified or abundances are not available, only the unweighted number is reported.

Fixes #1818.

TODO items

• consider tackling indicate threshold in sourmash gather output #1850, reporting threshold more clearly
• add a test for combined behavior, for both gather and multigather
• add a test for no gather result, for both gather and multigather

[codecov]

Codecov Report

Merging #2301 (8373c5c) into latest (719e7d5) will increase coverage by 7.31%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #2301      +/-   ##
==========================================
+ Coverage   84.83%   92.14%   +7.31%     
==========================================
  Files         131      100      -31     
  Lines       15676    11414    -4262     
  Branches     2252     2253       +1     
==========================================
- Hits        13298    10517    -2781     
+ Misses       2083      602    -1481     
  Partials      295      295              

Flag	Coverage Δ
python	92.14% <100.00%> (+<0.01%)	⬆️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/commands.py	90.25% <100.00%> (+0.12%)	⬆️
src/sourmash/search.py	97.95% <100.00%> (-0.01%)	⬇️
src/core/src/errors.rs
src/core/src/signature.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/sketch/minhash.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/cmd.rs
src/core/src/ffi/index/mod.rs
src/core/tests/storage.rs
... and 23 more

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[ctb]

Ready for review & merge @sourmash-bio/devs

[bluegenes]

lgtm! It'll be pretty helpful to have both weighted and unweighted results printed.