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. 
My point is more that you dont need to callback. You can imagine to have point,line,triangle in the class, and inherit to override the default beahvior. no ?