Add test for code to move over from GN1

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

* .gitignore: ignore emacs temporary files
* gn3/computations/correlations2.py: add a dummy function
* tests/unit/computations/test_correlation.py: add unit tests for the function

  As part of the move of the clustering and heatmap code over from GN1 to GN3,
  this commit begins by providing some unit tests for the correlation function
  used to ensure that the implementation that is built up here corresponds,
  and produces the same results as the original.

  This tests and the function might change in the new system, but for now, we
  try and maintain bug-to-bug compatibility.

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