[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 95.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.15%. Comparing base (5de4614) to head (4138743).

❗ There is a different number of reports uploaded between BASE (5de4614) and HEAD (4138743). Click for more details.

HEAD has 14 uploads less than BASE

Flag	BASE (5de4614)	HEAD (4138743)
	15	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #767       +/-   ##
===========================================
- Coverage   97.07%   85.15%   -11.92%     
===========================================
  Files         107      107               
  Lines       22138    22450      +312     
===========================================
- Hits        21490    19117     -2373     
- Misses        648     3333     +2685     

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