Refactor handling of coincident points in triangulation

[martinfleis]

Closes #713

Had to touch more stuff than intended but we now rely on QHull to identify coincident points and provide a lookup from which we can build cliques. This should ensure that we won't hit the precision issues.

Some minor cleanup happened alongside, i.e. to avoid solving isolates twice in a row.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.9%. Comparing base (bcabdbc) to head (4d7eb44).
Report is 16 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #714     +/-   ##
=======================================
- Coverage   85.0%   84.9%   -0.1%     
=======================================
  Files        141     145      +4     
  Lines      15203   15393    +190     
=======================================
+ Hits       12924   13075    +151     
- Misses      2279    2318     +39     

Files	Coverage Δ
libpysal/graph/_kernel.py	82.0% <100.0%> (-0.1%)	⬇️
libpysal/graph/_triangulation.py	98.0% <100.0%> (+0.1%)	⬆️
libpysal/graph/_utils.py	96.3% <100.0%> (+1.4%)	⬆️
libpysal/graph/base.py	97.0% <100.0%> (-1.0%)	⬇️
libpysal/graph/tests/test_builders.py	100.0% <100.0%> (ø)
libpysal/graph/tests/test_kernel.py	99.0% <100.0%> (+<0.1%)	⬆️
libpysal/graph/tests/test_triangulation.py	98.8% <100.0%> (+<0.1%)	⬆️

... and 12 files with indirect coverage changes

[martinfleis]

Okay, this is now significantly updated.

• switch terminology from coincident to coplanar as suggested by @ljwolf (cc @knaaptime in case you are using "coincident" somewhere in your materials, this is technically a breaking change but we're still in experimental mode)
• knn uses the same coplanar resolver as triangulation. that also required new function to build coplanar lookup to match the one coming from Delaunay
• raising CoplanarError
• always jitter when coplanar="jitter", no matter if there is a coplanar point or not

The performance is about the same as it was before.