Inner boundary: implement

[ThomasCeulemans]

Currently, we are treating all boundary points equally with our ray-tracer. (stop if ray tracing direction is in same direction of position with respect to model center). However, the inner boundary points are not guaranteed to lie around the center, and should always be te end of a ray. Therefore, I'll need to implement simple datafield to denote whether a boundary point is an inner boundary point or not.