Add cell_nodes property to TensorMesh

[santisoler]

Add a new cell_nodes property to TensorMesh that mimics the one of TreeMesh returning the indices of the nodes that correspond to each cell in the mesh. Add tests for the new feature.

[codecov]

Codecov Report

Merging #333 (1cf90d1) into main (58043b9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #333      +/-   ##
==========================================
+ Coverage   85.46%   85.47%   +0.01%     
==========================================
  Files          39       39              
  Lines       11990    12001      +11     
==========================================
+ Hits        10247    10258      +11     
  Misses       1743     1743              

Files Changed	Coverage Δ
discretize/tensor_mesh.py	85.25% <100.00%> (+0.53%)	⬆️

[jcapriot]

The implementation looks good!
The only thing would be that we should likely add a detail to the docstring (both here and in the TreeMesh while were at it) about the ordering of the nodes returned by the list, i.e.

2 -- 3
|    |
0 -- 1

For a 2D mesh.

[santisoler]

@jcapriot I just added some notes about the order of the nodes for each cell. Let me know if you like it.

The implementation looks good!

I just copied and pasted your implementation from the BasePFSimulation, so congrats to you haha.

[jcapriot]

Thanks, looks good to me know!