graphtool: plane doubleside prop

@kadst43 @AmbientOSX 

Added a property to set planes to either be double-sided or not. Also added in a method for setting properties of graphgroup meshes.
THREE.DoubleSide is a "constant" (even though no such thing exists in JavaScript). There's also THREE.FrontSide and THREE.BackSide. Ours is a boolean rather than an integer, so it should probably not be like-named to avoid confusion.