NumDiff abstraction for dynamics systems

We should provide an abstraction for computing the NumDiff, so the users can quickly test a system. NumDiff routines are needed to compute the linear approximation of the dynamics (see issue #19 (closed)). Having solved the issue #19 (closed) and #20 (closed), we can defined by default the updateLinearAppr by running a NumDiff routine.

The user can still deploy its analytical derivatives or potentially other methods such as automatic differentiation.