[MRG] add signature names to known/unknown hash sigs output by sourmash prefetch

[ctb]

When running genome-grist with the new picklist code over in dib-lab/genome-grist#94, I noticed that sourmash prefetch --save-matching-hashes and --save-unmatched-hashes created signatures with empty names, which then led to the downstream gather output for those signatures having no names, either. This fixes that to set the names to {query.name}-known and {query.name}-unknown respectively.

[codecov]

Codecov Report

Merging #1646 (a6801d5) into latest (e386085) will increase coverage by 7.40%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           latest    #1646      +/-   ##
==========================================
+ Coverage   81.92%   89.32%   +7.40%     
==========================================
  Files         112       85      -27     
  Lines       11441     7741    -3700     
  Branches     1430     1432       +2     
==========================================
- Hits         9373     6915    -2458     
+ Misses       1813      569    -1244     
- Partials      255      257       +2     

Flag	Coverage Δ
python	89.32% <75.00%> (-0.02%)	⬇️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/commands.py	86.23% <75.00%> (-0.15%)	⬇️
src/core/src/sketch/nodegraph.rs
src/core/src/index/linear.rs
src/core/src/wasm.rs
src/core/src/ffi/nodegraph.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/tests/minhash.rs
src/core/src/index/storage.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/index/sbt/mod.rs
... and 18 more

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[ctb]

anticipating that the remaining tests are going to pass... ready for review and merge, thx! @sourmash-bio/devs

[ctb]

thanks!