#7 Re-think the Arena stuffs from scratch

[Tomassino Ferrauto]

The implementation of the Arena and related classes is not scalable and not good in general. We should think of a way of doing fast 2D simulations which is more general. Perhaps this means modifying WObject to add functions to return different models (3D dynamic model, 3D kinematic model, 2D kinematic model, graphical representation...).