Potentials as tuple

[michalk8]

Also removes explicit passing of the dtype and the deprecated solve function.
Fixes some warnings and minor pet-peeves (like missing leading/trailing 0s).

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (a10c377) 90.58% compared to head (70c5495) 90.62%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #488      +/-   ##
==========================================
+ Coverage   90.58%   90.62%   +0.04%     
==========================================
  Files          60       60              
  Lines        6678     6678              
  Branches      952      954       +2     
==========================================
+ Hits         6049     6052       +3     
+ Misses        480      478       -2     
+ Partials      149      148       -1     

Files	Coverage Δ
src/ott/geometry/costs.py	92.92% <100.00%> (-0.02%)	⬇️
src/ott/geometry/geometry.py	92.69% <100.00%> (-0.03%)	⬇️
src/ott/geometry/graph.py	93.68% <100.00%> (ø)
src/ott/geometry/grid.py	91.53% <ø> (ø)
src/ott/geometry/low_rank.py	97.04% <100.00%> (ø)
src/ott/geometry/pointcloud.py	86.47% <100.00%> (ø)
src/ott/initializers/linear/initializers.py	100.00% <100.00%> (ø)
src/ott/initializers/linear/initializers_lr.py	91.38% <100.00%> (ø)
src/ott/math/matrix_square_root.py	100.00% <100.00%> (ø)
src/ott/math/unbalanced_functions.py	100.00% <100.00%> (ø)
... and 22 more