[MRG] rename containment to contained_by

[ctb]

We found that containment was confusing, so we renamed it to contained_by.

• 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 #199 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   86.22%   86.22%           
=======================================
  Files          13       13           
  Lines        1974     1974           
  Branches       52       52           
=======================================
  Hits         1702     1702           
  Misses        262      262           
  Partials       10       10

Impacted Files	Coverage Δ
sourmash_lib/commands.py	90.19% <100%> (ø)	⬆️
sourmash_lib/signature.py	93.1% <100%> (ø)	⬆️

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[ctb]

Ready for review & merge!

[luizirber]

Do you want to keep the spacegraphcats branch around?

[ctb]

yes, for a bit - thank you.