get_cells_along_line method not robust for tree mesh

[dccowan]

Summary

When the get_cells_along_line is called for a tree mesh, sometimes it will cause the kernel to crash. This seems to happen when a line segment is defined diagonally from one node to another, or one parallel edge to another. This has raised practical challenges in EM modeling with SimPEG. I have attached files to reproduce the error directly with discretize, and for a TDEM simulation in SimPEG. The get_cells_along_line method is defined in discretize/_extensions/tree_ext.pyx.

test_files.zip

For this issue to be fixed, the scripts should run correctly without failing for both values of the variable 'height' specified.

[dccowan]

Unhappy cases