Add sparse Chebyshev approximation

[guillaumehu]

In this PR, we modify the Chebyshev approximation used in the geodesic geometry to work with sparse BCOO graph. This PR would close #476.

• Sparse Laplacian fn
• Sparse Cheb approximation fn
• test heat kernel approx
• test that it integrates with Sinkhorn.

Questions
Do we want to test that it is faster for sparse matrices ? We added a test for the memory usage.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.61%. Comparing base (da704fc) to head (e9bc344).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #502      +/-   ##
==========================================
+ Coverage   90.59%   90.61%   +0.02%     
==========================================
  Files          60       60              
  Lines        6688     6703      +15     
  Branches      956      958       +2     
==========================================
+ Hits         6059     6074      +15     
  Misses        480      480              
  Partials      149      149              

Files	Coverage Δ
src/ott/geometry/geodesic.py	93.33% <100.00%> (+1.11%)	⬆️

[guillaumehu]

Thank you for reviewing the PR @michalk8 ! I made the suggested changes, let me know what you think.

[michalk8]

Thanks a lot @guillaumehu , LGTM!