move out new_ds to arviz-stats

[aloctavodia]

Depends on arviz-devs/arviz-stats#38

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📚 Documentation preview 📚: https://arviz-plots--102.org.readthedocs.build/en/102/

[aloctavodia]

Tests are failing because with the changes in this PR the plot depends on a function that needs to be imported from arviz-stats, the function is there but not in the release version we are using for tests.
Probably the easiest thing to do is just wait until a next release.

[OriolAbril]

We can modify the dependencies to install arviz-stats from github:

arviz-plots/pyproject.toml

Line 30 in 2758b92

"arviz-stats[xarray] @ git+https://github.com/arviz-devs/arviz-stats",

(which we have done multiple times). Otherwise we'd have to either release constantly or end up developing very slowly

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.04%. Comparing base (60ca1bc) to head (dc5169f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #102      +/-   ##
==========================================
- Coverage   86.09%   86.04%   -0.05%     
==========================================
  Files          24       24              
  Lines        2855     2845      -10     
==========================================
- Hits         2458     2448      -10     
  Misses        397      397              

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