Implement the method validate in all the potentials

[Jorge Pérez]

This method is currently only “implemented” in the parent, and always return true. It should be mandatory to implement the method "validate" for all potentials (for example, by forcing the method to be abstract in the class "Potential"). This method must check if the potential is valid.

In spite this method is usefull for all the potentials, the implementation could be time-consuming for the group and it is possible that we can start the implementation with one or two high-priority potentials, letting others potentials to return the “NotImplementedException”.

I assigned this issue to @Francisco Javier Díez so he can evaluate the priority and which resources must be allocated.