[MRG] Distance calculation bug solve

[ncassereau]

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Motivation and context / Related issue

Resolves #305, also allows the usage of weights (otherwise wminkowski is not a valid metric contrary to what POT doc suggests)

How has this been tested (if it applies)

Tests ran locally without a hitch

Checklist

• The documentation is up-to-date with the changes I made.
• I have read the CONTRIBUTING document.
• All tests passed, and additional code has been covered with new tests.

[codecov]

Codecov Report

Merging #306 (4de4f54) into master (f162879) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #306      +/-   ##
==========================================
+ Coverage   93.38%   93.43%   +0.04%     
==========================================
  Files          21       21              
  Lines        4886     4888       +2     
==========================================
+ Hits         4563     4567       +4     
+ Misses        323      321       -2