[MRG] Fix Jaccard calculation to be intersection over union

[ctb]

Fixes #125 so that we are MinHash-correct by computing Jaccard estimator as intersection over union of k-mers. This makes it symmetric & presumably mash compatible but also breaks many tests :)

• 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?

TODO:

• confirm that the default behavior is mash compatible

[ctb]

@luizirber I ran headlong into Cython issues related to calling merge on KmerMinHash objects. I would like to solicit your opinions on my changes :)

[ctb]

@luizirber all tests fixed. Review pls?

[codecov-io]

Codecov Report

Merging #150 into master will decrease coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #150      +/-   ##
==========================================
- Coverage   85.52%   85.31%   -0.22%     
==========================================
  Files          13       13              
  Lines        1879     1879              
  Branches       52       52              
==========================================
- Hits         1607     1603       -4     
- Misses        263      265       +2     
- Partials        9       11       +2

Impacted Files	Coverage Δ
sourmash_lib/kmer_min_hash.hh	88% <100%> (-2.29%)	⬇️

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[luizirber]

LGTM. We shouldn't need to change the merge to merge_abund, but it's not a problem that needs fixing in this PR.