New Intersection stuff

Replaced the old IntersectVisitor with a new IntersectionVisitor which gives support to add different types of intersector. 

The LineSegmentIntersector replaces the full functionality that the old intersector had. Including KD-trees. 

The PolytopeIntersector mimics the OSG one, now the user can 'pick' points and lines, and also triangles.

Added a PrimitiveFunctor class to get access to the primitives of a Geometry, something like the TemplatePrimitiveFunctor class in OSG. 
Same as above. The idea is to use .create() or only if it's really necessary (basically when we are doing this._candidates.push(**here we can clone/create**);