Faster node density

[u3ks]

I moved the node density functionality to the graph module and use networkx to compute it.
The current implementation scales horribly and on larger graphs takes hours, since all edges have to be filtered multiple times with np.in1d.
Getting the exact timing is a bit messy since subgraphs and higher order pysal have to be precomuted and affect the time.
The new implementation takes around <20ish mintues on a graph with ~500k nodes and 2mil edges, together with the other mm.subgraph computations.
For smaller datasets the current implementation + calculating a higher order graph ~ the new networkx implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.0%. Comparing base (4037c70) to head (9ed20b3).
Report is 47 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #609     +/-   ##
=======================================
+ Coverage   97.4%   98.0%   +0.6%     
=======================================
  Files         26      37     +11     
  Lines       4328    6079   +1751     
=======================================
+ Hits        4214    5957   +1743     
- Misses       114     122      +8     

Files	Coverage Δ
momepy/functional/_intensity.py	100.0% <100.0%> (ø)
momepy/functional/tests/test_intensity.py	100.0% <100.0%> (ø)
momepy/graph.py	98.9% <100.0%> (+0.1%)	⬆️

[martinfleis]

Cool!

[martinfleis]

One more thing, can you update API docs? Removing the old one and adding the new to the proper section.