fix point_clouds.ipynb

[marcocuturi]

New tutorial for point_clouds,

• simplified with less calls to soon to be deprecated APIs, and more reliance on the geometry, LinearProblem and Sinkhorn solver classes.
• simplified "optimize" loop, 1 call to Sinkhorn instead of 2 previously.
• more examples of gradient flows.

[review-notebook-app]

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[codecov-commenter]

Codecov Report

Merging #159 (e6524b3) into main (8646692) will decrease coverage by 5.86%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #159      +/-   ##
==========================================
- Coverage   89.83%   83.97%   -5.87%     
==========================================
  Files          51       51              
  Lines        5068     5068              
  Branches      519      519              
==========================================
- Hits         4553     4256     -297     
- Misses        391      660     +269     
- Partials      124      152      +28     

Impacted Files	Coverage Δ
ott/core/discrete_barycenter.py	15.58% <0.00%> (-70.13%)	⬇️
ott/tools/segment_sinkhorn.py	43.75% <0.00%> (-56.25%)	⬇️
ott/tools/sinkhorn_divergence.py	61.81% <0.00%> (-32.73%)	⬇️
ott/tools/soft_sort.py	69.90% <0.00%> (-25.25%)	⬇️
ott/core/segment.py	75.00% <0.00%> (-25.00%)	⬇️
ott/core/gw_barycenter.py	74.50% <0.00%> (-22.55%)	⬇️
ott/geometry/pointcloud.py	68.62% <0.00%> (-14.96%)	⬇️
ott/geometry/graph.py	81.69% <0.00%> (-12.68%)	⬇️
ott/core/bar_problems.py	74.57% <0.00%> (-9.61%)	⬇️
...tt/tools/gaussian_mixture/gaussian_mixture_pair.py	85.33% <0.00%> (-6.67%)	⬇️
... and 20 more

[marcocuturi]

failed test seems to come from bug in CI.