Add const on double* in vtkBox::IntersectBox and int* version for vtkPolygon::ComputeNormal

• const double* parameter instead of double* in vtkBox::IntersectBox
• vtkCell3D::GetFacePoints writes point ids as int*, and vtkPolygon::ComputeNormal takes ids as vtkIdType*, which makes it impossible to use without first copying the array int[] into another array vtkIdType[]. This MR adds a version of vtkPolygon::ComputeNormal with int* parameter, as well as an overload handling std::nullptr_t parameters, as the original method did handle a nullptr input.