Add grasshopper parameter for plankton mesh

* 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)
Merge pull request #13 from pearswj/12-gh-param

Add grasshopper parameter for plankton mesh