[WIP] Sliced OT Plans

[eloitanguy]

Types of changes

Note: this PR continues #757 which was merged by accident.

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:

• update PR number in RELEASES.md and README.md
• 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

❌ Patch coverage is 80.32258% with 61 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.51%. Comparing base (6e21613) to head (43dd522).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #767      +/-   ##
==========================================
- Coverage   96.78%   96.51%   -0.28%     
==========================================
  Files         118      121       +3     
  Lines       23721    23391     -330     
==========================================
- Hits        22958    22575     -383     
- Misses        763      816      +53     

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[clbonet]

Nice PR @eloitanguy @lchapel!
Here are few comments.