[MRG] correct independence of fgw barycenters to init

[cedricvincentcuaz]

Types of changes

• correct independence of fgw barycenters to init by changing performing the 1st barycenter update after the computing the 1st OT matrices.
• Removed some unnecessary operations when fixed_features=True and/or warmstart=False.

Motivation and context / Related issue

• Cf Issue fgw barycenter often returns matrix with same entries #547

How has this been tested (if it applies)

• already handled by existing tests.

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 #566 (882202c) into master (1682b60) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #566      +/-   ##
==========================================
+ Coverage   96.49%   96.51%   +0.01%     
==========================================
  Files          67       67              
  Lines       14816    14821       +5     
==========================================
+ Hits        14297    14304       +7     
+ Misses        519      517       -2     

[rflamary]

Thanks @cedricvincentcuaz a few comments and we can merge

[cedricvincentcuaz]

Thank you for your feedback @rflamary. I took into account your comments.