added computeNormal() function

Signed-off-by: Marcel Schwittlick <marzzzel@gmail.com>
changed to pure virtual interface

Signed-off-by: Marcel Schwittlick <marzzzel@gmail.com>
wrote class signature. too tired to implement.

Signed-off-by: Marcel Schwittlick <marzzzel@gmail.com>
implemented TriangleIntersector class

Signed-off-by: Marcel Schwittlick <marzzzel@gmail.com>
implemented Intersector3D interface as pure virtual

Signed-off-by: Marcel Schwittlick <marzzzel@gmail.com>