Putting computation of forces and moments in a separate class named ContactModel
Changing method for measuring computation time
Revert "Putting computation of forces and moments in a separate class named ContactModel"

This reverts commit af03a235ea7cc8a12eabbd7135bf86ab095ab242.
adding elasting pendulum contact model. We can now choose between two models
correcting a bug in the elast pendulum model and adding the method that permit to choose the contact model in the entity