Compute bounding sphere for Skin node

[stefaniapedrazzi]

We should add the computation of the bounding sphere for the Skin node in order to enable the "Move viewpoint to object feature".

To make that the Skin node is visible to Pen and infra-red Distance Sensor we also have to implement the pickUVCoordinate method.

[stefaniapedrazzi]

Next step is to compute the pickUVCoordinate, collisionDistance and collisionLocalPoint functions for Skin node to let the Pen and infra-red DistanceSensor work with the skin.