[MRG] refactor and simplify sourmash sig subcommand signature loading

[ctb]

Fixes #1631

• adds --force and --from-file to most of sig commands.
• adds -k and moltype arguments to most of the sig commands.
• supports more robust loading of signatures (and catching of errors); better UX, too.
• substantial code simplification and refactoring.

TODO:

• test split --from-file
• test downsample --num on scaled sig
• test describe --from-file
• test merge --from-file
• test intersect --from-file
• test rename --from-file
• test extract --from-file
• test flatten --from-file
• test downsample --num --scaled both
• revisit LoadManySignatures - should it be a generator fn only? should it be in sig/__main__.py only?
• probably worth writing some tests for LoadManySignatures directly.
• add picklists throughout?

[codecov]

Codecov Report

Merging #1672 (b23e54c) into latest (b16930a) will increase coverage by 7.56%.
The diff coverage is 97.93%.

@@            Coverage Diff             @@
##           latest    #1672      +/-   ##
==========================================
+ Coverage   82.42%   89.99%   +7.56%     
==========================================
  Files         113       86      -27     
  Lines       11847     8173    -3674     
  Branches     1507     1508       +1     
==========================================
- Hits         9765     7355    -2410     
+ Misses       1822      561    -1261     
+ Partials      260      257       -3     

Flag	Coverage Δ
python	89.99% <97.93%> (+0.19%)	⬆️
rust	?

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

Impacted Files	Coverage Δ
src/sourmash/cli/categorize.py	100.00% <ø> (ø)
src/sourmash/cli/compute.py	100.00% <ø> (ø)
src/sourmash/cli/import_csv.py	100.00% <ø> (ø)
src/sourmash/cli/sig/export.py	100.00% <ø> (ø)
src/sourmash/cli/sig/filter.py	100.00% <ø> (ø)
src/sourmash/cli/sig/ingest.py	100.00% <ø> (ø)
src/sourmash/cli/sig/manifest.py	100.00% <ø> (ø)
src/sourmash/sourmash_args.py	93.54% <91.30%> (-0.12%)	⬇️
src/sourmash/sig/__main__.py	93.89% <98.25%> (+2.90%)	⬆️
src/sourmash/cli/sig/cat.py	100.00% <100.00%> (ø)
... and 36 more

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[ctb]

Ready for review and merge @sourmash-bio/devs!

[bluegenes]

I think this could use picklist tests for cat and subtract...?

[bluegenes]

lgtm