RK4

I would like that we reintroduce the RK4 in the refact branch, and compare it with impl-eul. This means defining a new IntegratedActionModel, based on the impl-eul class. We should have one DifferentialActionData (DAD) per integration point (each time you call a DAM.calc, it should be on a different DAD). The calc function should just be a copy paste of the devel branch. The calcDiff can be discussed. Basically, the idea is to call the DAM.calcDiff on every integration point, and then make the same operation on the calcDiff matrices that were done on the calc vectors.

Let's not think of RK45 (variable-time integration) yet.