[MRG] Gaussian Gromov wasserstein solvers

[rflamary]

Types of changes

• Implement Gaussian GW loss
• Implement Gaussian GW mapping
• Implement DA mapping between spaces with gaussian Gromov (Closes Linear OT mapping across different spaces #271)
• Documentation and examples (update linear mapping exmaple)
• Do all tests

Updates plots in the linear mappinge xmaple:

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.

[codecov]

Codecov Report

Merging #498 (bde1e19) into master (2bbfbbb) will increase coverage by 0.70%.
Report is 30 commits behind head on master.
The diff coverage is 95.80%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #498      +/-   ##
==========================================
+ Coverage   94.92%   95.62%   +0.70%     
==========================================
  Files          31       65      +34     
  Lines        6879    13588    +6709     
==========================================
+ Hits         6530    12994    +6464     
- Misses        349      594     +245     

[cedricvincentcuaz]

Hello @rflamary, very nice PR as usual.
I just had few remarks on the documentation.