[MRG] srFGW barycenters

[cedricvincentcuaz]

Types of changes

• add srFGW barycenters semirelaxed_fgw_barycenters with similar API than fgw_barycenters
• add generic function for update of barycenter structure and features in gromov/_utils: update_barycenter_structure and update_barycenter_feature which extend/relax several aspects of existing functions for gw barycenters (update_square_loss, update_kl_loss and update_feature_matrix --> removed), seeking for potential use in other variants than gw, e.g now used with sr(F)GW but could be also used with P(F)GW and U(F)GW.

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

All modified and coverable lines are covered by tests ✅

Project coverage is 96.71%. Comparing base (d0849a4) to head (e5afab2).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #659      +/-   ##
==========================================
+ Coverage   96.67%   96.71%   +0.03%     
==========================================
  Files          85       86       +1     
  Lines       16956    17148     +192     
==========================================
+ Hits        16392    16584     +192     
  Misses        564      564