Write a function to get the brigtest path from a spine to a segment

[cudmore]

TODO: @jtle00 and @Suhayb-A

If we write an atomic function to simply get the brightest path from a spine (x,y,z) to a segment, we can move forward.

See our _findBrightestIndex which basically does th job. It will need to be modified (of course) but it can be added to the backend as an atomic function in, for example, a util.py file?

Let me know what you think?