[MRG] OT barycenters for generic transport costs

[eloitanguy]

Types of changes

• contribute a generalisation of free_support that accepts any cost function (implements this paper)
• contribute a specific version of this function in ot.gmm for fast computation of GMM barycenters
• example for new barycenter function
• example of GMM barycenters
• add summary of features and example in README.md

Motivation and context / Related issue

How has this been tested (if it applies)

• full coverage in test/test_ot.py and test/test_gmm.py

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

All modified and coverable lines are covered by tests ✅

Project coverage is 97.10%. Comparing base (a11b055) to head (0820e51).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #715      +/-   ##
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+ Coverage   97.08%   97.10%   +0.01%     
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  Files         100      100              
  Lines       19947    20081     +134     
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+ Hits        19365    19499     +134     
  Misses        582      582