Flexible Core Library

The goal is to make the *core* Plankton library as flexible as possible by removing all references to the Grasshopper/Rhino assemblies. Instead helper functions are provided (mostly in the form of extension methods) in the Grasshopper library, *PlanktonGh*.

For example, instead of the old `PlanktonMesh(Rhino.Geometry.Mesh)` constructor you can call the `ToPlanktonMesh()` extension on any instance of Rhino's `Mesh`. For this to work you must also reference the Plankton *GHA* in your C#/VB scripts.

This builds on some great ideas in [piac/TurtleMesh](http://github.com/piac/TurtleMesh).