[WIP] Sliced OT Plans

[eloitanguy]

Types of changes

Implements min-Pivot Sliced Plans Mahey et al., Tanguy et al. and Expected Sliced Plans Liu et al., Tanguy et al., and Generalised Sliced Wasserstein Plans Chapel et al..

Checklist:

• implement min-Pivot Sliced
• implement Expected Sliced: todo: raise error when using tf or jax, test accordingly, and add a warning in the doc
• example
• testing (min-Pivot Sliced and Expected Sliced)
• discuss API choices
• Generalised SW plans implementation + example
• doc proofreading (min-Pivot Sliced and Expected Sliced)

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.18%. Comparing base (85113e9) to head (08c5348).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #757 +/- ## ========================================== + Coverage 97.15% 97.18% +0.03%  ========================================== Files 107 107 Lines 21877 22035 +158 ========================================== + Hits 21254 21415 +161  + Misses 623 620 -3 

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