Add function for converting concentration map to tortuosity

[jgostick]

This was written in Julia, but I think a python version is kicking around too?

@ma-sadeghi, do you have this handy?

[jgostick]

We already have this in the beta module, just need to move it.

[ma-sadeghi]

@jgostick IIRC, there was an edge-case bug that you discovered a while back that I was supposed to fix. Do you happen to remember what the edge case was?