Gradient look-up

Implemented gradient look-up from the river slope map this week, which lets us determine where a newly generated node should be positioned. I also added a horizontal jitter to handle the case where a parent node splits off into two children nodes with the same starting point and gradient vector. The sample river slope map I'm working off of at the moment is a simple radial gradient (lighter values designate higher elevation).

Nodes seem to be coming along nicely. Next steps are to implement geometry compatibility checks to determine when expansion should terminate. After that I'll be working on defining different river types and adding that to the final output.