[MRG] Add factored coupling

[rflamary]

Types of changes

This PR aim at adding the factored coupling solver of:

Forrow, A., Hütter, J. C., Nitzan, M., Rigollet, P., Schiebinger,
G., & Weed, J. (2019, April). Statistical optimal transport via factored
couplings. In The 22nd International Conference on Artificial
Intelligence and Statistics (pp. 2454-2465). PMLR.

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 #358 (46b8dd7) into master (7671715) will increase coverage by 0.05%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #358      +/-   ##
==========================================
+ Coverage   93.69%   93.74%   +0.05%     
==========================================
  Files          22       23       +1     
  Lines        5753     5799      +46     
==========================================
+ Hits         5390     5436      +46     
  Misses        363      363