[MRG] Apply downsampling magic to sbt_search

[ctb]

Fixes #146 (a bug in sbt_search). Also fixes #182 (--containment option in search).

• 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-io]

Codecov Report

Merging #183 into master will increase coverage by 0.15%.
The diff coverage is 84.09%.

@@            Coverage Diff             @@
##           master     #183      +/-   ##
==========================================
+ Coverage   85.67%   85.83%   +0.15%     
==========================================
  Files          13       13              
  Lines        1906     1934      +28     
  Branches       52       52              
==========================================
+ Hits         1633     1660      +27     
- Misses        262      264       +2     
+ Partials       11       10       -1

Impacted Files	Coverage Δ
sourmash_lib/commands.py	90.75% <100%> (+0.66%)	⬆️
sourmash_lib/signature.py	89.6% <100%> (+0.48%)	⬆️
sourmash_lib/sbtmh.py	87.91% <65%> (-6.83%)	⬇️
sourmash_lib/kmer_min_hash.hh	89.14% <0%> (+1.14%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3cd0fe2...211a994. Read the comment docs.

[ctb]

Ready for review & merge @luizirber @betatim

[luizirber]

👍