[MRG] implement sourmash sketch fromfile

[ctb]

Note: PR into #1907.

This PR creates a sourmash sketch fromfile command to support large-scale/batch construction of signatures.

Addresses #1671.
Fixes #1634.

TODO:

• add docs for fromfile
• consider removing 'identprefix' column from fromfile format? or just say 'other columns ignored'
• check license / CC0 stuff
• check-sequence stuff - valid DNA?
• add tests for --output-csv
• do an end-to-end test with assembly summary files Building genbank/refseq databases from assembly_summary.txt databases#7
• block --seed?
• add picklist/manifest output for already-existing sigs that match param spec
• add simple human-readable summarization output
• how does --singleton interact with fromfile? not currently implemented? consider implementing sourmash sketch fromfile --singleton mode. #1919
• resolve @ CTB stuff
• refactor and cleanup once code is under test

[codecov]

Codecov Report

Merging #1885 (4f43d9e) into latest (d8102df) will increase coverage by 0.26%.
The diff coverage is 98.34%.

@@            Coverage Diff             @@
##           latest    #1885      +/-   ##
==========================================
+ Coverage   82.96%   83.22%   +0.26%     
==========================================
  Files         123      124       +1     
  Lines       13367    13601     +234     
  Branches     1808     1861      +53     
==========================================
+ Hits        11090    11320     +230     
- Misses       2013     2014       +1     
- Partials      264      267       +3     

Flag	Coverage Δ
python	91.02% <98.34%> (+0.18%)	⬆️
rust	65.80% <ø> (ø)

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

Impacted Files	Coverage Δ
src/sourmash/cli/sketch/dna.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/protein.py	100.00% <ø> (ø)
src/sourmash/cli/sketch/translate.py	100.00% <ø> (ø)
src/sourmash/command_sketch.py	93.29% <97.96%> (+5.09%)	⬆️
src/sourmash/cli/sketch/__init__.py	100.00% <100.00%> (ø)
src/sourmash/cli/sketch/fromfile.py	100.00% <100.00%> (ø)
src/sourmash/command_compute.py	92.87% <100.00%> (+0.42%)	⬆️

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[ctb]

ok, I think the functionality here is largely done and at a good resting place. It's a bit of a monster PR and while I do have ideas on where to take the functionality next (AS DOES TESSA I'M SURE), I think it might be good to get this reviewed.

So, ready for review @sourmash-bio/devs !

[bluegenes]

🎉 🎉