add bar

[marcocuturi]

adds barycenter operator for NegDotProduct(). Same as sq. Euclidean distance

[michalk8]

LGTM, thanks!

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 87.34%. Comparing base (afdd39b) to head (739f347).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/ott/geometry/costs.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #670      +/-   ##
==========================================
- Coverage   87.35%   87.34%   -0.01%     
==========================================
  Files          82       82              
  Lines        8478     8480       +2     
  Branches      581      581              
==========================================
+ Hits         7406     7407       +1     
- Misses        922      923       +1     
  Partials      150      150              

Files with missing lines	Coverage Δ
src/ott/geometry/costs.py	96.54% <50.00%> (-0.25%)	⬇️

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