[MRG] alias sig fileinfo to sig summarize; minor docs, test cleanup

[ctb]

This PR implements several minor cleanup and feature additions:

• aliases sourmash sig fileinfo to sourmash sig summarize per provide a summarization command for signature collections #1439
• upgrades manifest and picklist documentation in the command-line docs
• provides better manifest and fileinfo CLI usage help
• does some pyflakes-based cleanup of tests, including rescuing three shadowed tests from copypasta mistakes

Fixes #1439

[codecov]

Codecov Report

Merging #1863 (de073c0) into latest (4defa47) will increase coverage by 7.98%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1863      +/-   ##
==========================================
+ Coverage   82.36%   90.34%   +7.98%     
==========================================
  Files         119       89      -30     
  Lines       12929     8732    -4197     
  Branches     1727     1727              
==========================================
- Hits        10649     7889    -2760     
+ Misses       2016      579    -1437     
  Partials      264      264              

Flag	Coverage Δ
python	90.34% <100.00%> (+<0.01%)	⬆️
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/cli/sig/__init__.py	100.00% <100.00%> (ø)
src/sourmash/cli/sig/fileinfo.py	100.00% <100.00%> (ø)
src/sourmash/cli/sig/manifest.py	100.00% <100.00%> (ø)
src/sourmash/sig/__main__.py	91.78% <100.00%> (+0.02%)	⬆️
src/core/src/index/sbt/mod.rs
src/core/src/index/search.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/ffi/index/revindex.rs
src/core/src/cmd.rs
src/core/src/sketch/nodegraph.rs
... and 24 more

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[ctb]

Ready for review & merge @sourmash-bio/devs

[bluegenes]

lgtm!