Python pred pray grass implementation

[Robadob]

Potentially useful here in future. It has some unused vec3 util functions (left over from the original boids model) and Ensemble code but may be useful until we fully add python support to the tutorial.

https://gist.github.com/jcbpyle/4244674a039d3d32b53a0627a2efb412

cc @jcbpyle