[MRG] add ksize selectors back into sourmash sig functions

[ctb]

Add -k back into sourmash sig functions.

Fixes #1104

TODO: add tests for

• merge
• intersect
• subtract
• rename
• extract
• filter
• flatten

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• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1105 into master will increase coverage by 9.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1105      +/-   ##
==========================================
+ Coverage   83.86%   92.90%   +9.04%     
==========================================
  Files          98       74      -24     
  Lines        9108     5736    -3372     
==========================================
- Hits         7638     5329    -2309     
+ Misses       1470      407    -1063     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
sourmash/sig/__main__.py	93.40% <100.00%> (+0.14%)	⬆️
sourmash/signature.py	92.85% <0.00%> (-0.27%)	⬇️
sourmash/_minhash.py	94.82% <0.00%> (-0.26%)	⬇️
sourmash/command_compute.py	97.44% <0.00%> (-0.20%)	⬇️
sourmash/sbtmh.py	89.71% <0.00%> (-0.19%)	⬇️
sourmash/nodegraph.py	93.57% <0.00%> (-0.18%)	⬇️
sourmash/sbt_storage.py	92.00% <0.00%> (-0.12%)	⬇️
sourmash/index.py	97.61% <0.00%> (-0.09%)	⬇️
sourmash/sbt.py	88.79% <0.00%> (-0.07%)	⬇️
sourmash/lca/lca_db.py	97.17% <0.00%> (-0.05%)	⬇️
... and 25 more

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[ctb]

Ready for review and merge! @erikyoung85 can you take a look at this? :)

[erikyoung85]

lgtm!

[erikyoung85]

Actually there seems to be an issue with building sourmash

[ctb]

ooh yes, a test failed on the Travis CI build! will fix.