Snowflake growth simulation in BlocksCAD

**Update:** For a direct OpenSCAD implementation of the algorithm, see [here](https://www.thingiverse.com/thing:3270304).

This implements a modification of [Reiter](http://patarnott.com/pdf/SnowCrystalGrowth.pdf)'s hex cell snowflake growth algorithm in BlocksCAD. The algorithm uses a real-valued cellular automaton modeling the diffusion and accretion of moisture. When run at high resolution, the model can produce surprisingly lifelike snowflakes, but BlocksCAD can't handle more than about ra