[MRG] add sig cat --from-file

[ctb]

Adds --from-file to sig cat per #1631.

[codecov]

Codecov Report

Merging #1657 (e98d2d3) into latest (97fa790) will increase coverage by 7.41%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1657      +/-   ##
==========================================
+ Coverage   82.22%   89.64%   +7.41%     
==========================================
  Files         113       86      -27     
  Lines       11704     7995    -3709     
  Branches     1478     1479       +1     
==========================================
- Hits         9624     7167    -2457     
+ Misses       1820      568    -1252     
  Partials      260      260              

Flag	Coverage Δ
python	89.64% <100.00%> (+<0.01%)	⬆️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/cli/sig/cat.py	100.00% <100.00%> (ø)
src/sourmash/sig/__main__.py	90.98% <100.00%> (+0.07%)	⬆️
src/sourmash/sourmash_args.py	93.43% <100.00%> (ø)
src/core/src/ffi/minhash.rs
src/core/src/index/search.rs
src/core/src/signature.rs
src/core/tests/test.rs
src/core/src/index/sbt/mod.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/index/storage.rs
... and 20 more

---

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[ctb]

Ready for review & merge @sourmash-bio/devs

[bluegenes]

lgtm!

[bluegenes]

Closes #1631