Core: ShapeModel parameter n_subdiv is never used

The parameter ShapeModelParameters::n_subdiv is an integer to determine the grid to subdivide a pixel when constructing the ShapeModel in order to get a smoother shape. However, it is only ever passed as an optional argument in the constructor ShapeIntegrator::ShapeIntegrator(ShapeModel* lib, const AABB& aabb, int nx, int ny, int nz, int subdiv = 1). This option is never used or modified, and as such, the number of subdivisions is always set to 1. When this value is changed, the code crashes.