[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 98.59155% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.90%. Comparing base (9f43898) to head (659e5fc).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #767      +/-   ##
==========================================
+ Coverage   96.87%   96.90%   +0.02%     
==========================================
  Files         113      113              
  Lines       23062    23412     +350     
==========================================
+ Hits        22342    22687     +345     
- Misses        720      725       +5     

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.