updated snow plow STL

Not sure why I never committed/pushed this.
remove unnecessary comments
add parameters to render rails onto track
Fix non-manifold edge
A single module for rendering all std track types

* Curved
* Straight
* Combinations (e.g. Y, switch-right, trident)

There are a LOT of permutations so it will take me awhile
to render all of the STL files.