Add tests for ordering and implement function

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

* gn3/computations/heatmap.py: implement new ordering function
* tests/unit/computations/test_heatmap.py: add new tests

  Implement the ordering function to migrate the setup of the `neworder`
  variable from GN1 to GN3.

  This migration is incomplete, since there is dependence on the return from
  the `web.webqtl.heatmap.Heatmap.draw` function in form of the `d_1` variable
  in some of the paths.

  The thing is, this `d_1` variable, and the `xoffset` variable seem to be
  used for laying out things on the drawn heatmap, and might actually end up
  not being needed for the new system using plotly, which has other ways of
  laying out things on the drawing.

  For now though, this commit "shims" the presence of these values until when
  the use of these variables is confirmed as present or absent in the new GN3
  system.

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