[WIP] Gaussian Mixture OT

[eloitanguy]

TODO

• [ ] make proj_SDP batch-friendy and backend-compatible
• [ ] finish flow example
• [ ] re-implement pdfs
• [ ] documentation
• [ ] faster bures distance matrix (re-implement bures and store square roots): einsum?
• [ ] faster barycentric mapping of GMM plan (same idea as for bures)
• [ ] stochastic GMM mapping
• [ ] asserts / tests

Types of changes

Motivation and context / Related issue

How has this been tested (if it applies)

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.