[MRG] Add method argument to sinkhorn Transport

[tgnassou]

Types of changes

Add parameter method to Sinkhrontransport int order to choose the method wanted for sinkhorn.

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 #440 (9f0925b) into master (a313e21) will increase coverage by 0.00%.
The diff coverage is 100.00%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #440   +/-   ##
=======================================
  Coverage   94.70%   94.70%           
=======================================
  Files          24       24           
  Lines        6608     6609    +1     
=======================================
+ Hits         6258     6259    +1     
  Misses        350      350