[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 97.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.58%. Comparing base (5a7d086) to head (2f4b675).

❗ There is a different number of reports uploaded between BASE (5a7d086) and HEAD (2f4b675). Click for more details.

HEAD has 14 uploads less than BASE

Flag	BASE (5a7d086)	HEAD (2f4b675)
	15	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #767       +/-   ##
===========================================
- Coverage   97.15%   85.58%   -11.57%     
===========================================
  Files         107      107               
  Lines       21877    22169      +292     
===========================================
- Hits        21254    18974     -2280     
- Misses        623     3195     +2572     

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