Renaming of abstract methods for defining a system

An user should be able to change only two main things:

1. the computation of f(x,v,u), and
2. the linear approximation of f(x,v,u).

The current names are forwardCalc and backwardCalc. This names have a sense for DDP but no meaning inside the concept of the dynamics. Indeed DDP during its forward-pass needs to update the dynamics and its backward-pass to update the linear approximation of the dynamics.

Therefore these methods should be renamed as:

1. updateDynamics
2. updateLinearAppr