[MRG] Add weak OT solver

[rflamary]

Types of changes

• Add weak OT solver
• Add tests for solver and backends
• Add example for weak OT in doc

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 #341 (effe32f) into master (71a57c6) will increase coverage by 0.15%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #341      +/-   ##
==========================================
+ Coverage   93.08%   93.23%   +0.15%     
==========================================
  Files          21       22       +1     
  Lines        5218     5250      +32     
==========================================
+ Hits         4857     4895      +38     
+ Misses        361      355       -6     

[ncourty]

thx for the nice work rémi !