Add grasshopper parameter for plankton mesh

Resolves #12. Details below...

* all credit to Giulio Piacentino and his TurtleMesh project for the param implemetation
    * automatically casts from Rhino mesh
    * native preview
    * reference mesh in Rhino
    * NO serialisation yet
* makes GHMeshToPMesh component obsolete
* rough and ready PlanktonMesh copy constructor (needed for param)

![image](https://f.cloud.github.com/assets/121068/2040561/8c424592-89b8-11e3-8933-72ab2a228bb6.png)