NumDiff abstraction and unit-test

In this PR I have included the unit-test for checking analytical differentiation of spring-mass and forward-dynamics systems. For that, I have to work in the NumDiff abstraction for whatever system. It also allows the user to start developing a system without the needed of adding the system derivatives. All this is possible because of the new API for dynamics and cost functions. In few works, the state and control vectors aren't included in the dynamics data.

This PR tackles the task #21 (closed) and partially the task #36 (closed).