Implement `control_samples` function as is in GN1

Issue:
https://github.com/genenetwork/gn-gemtext-threads/blob/main/topics/gn1-migration-to-gn2/partial-correlations.gmi

* gn3/partial_correlations.py: Implement `control_samples` function
* tests/unit/test_partial_correlations.py: add tests for `control_samples`
  function

  Implement the function `control_samples` and make it mostly bug-compatible
  with the `web/webqtl/correlation/correlationFunction.controlStrain` function
  in GN1.

  This implementation in GN3 does not do any calls to the database. It will
  rely on other functions to provide the data from the database to it.

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