Created an spring-mass system for unit-test

I added the spring-mass dynamics. However this task involves to improve the code in such a way that it's possible to reuse code. Here the list of changes:

1. Created abstract functions for the difference and integrate operators (dynamics operator) (2e21de4d)
2. Created a data for the acceleration function which allows us to reuse the integrator (91a7d8a4, 0521d10a)
3. Setting quadratic weights for scalar cases (490478ef)
4. Integrator object belongs to the dynamics (7cbe1eaa)
5. Changed the integrator API, using the dynamics data and model (344f47f7)

Note that:

• Point 1 improves #26 (closed).
• Point 2 and 4 allows us to generalize them, it solves #30 (closed).
• Point 5 solves #25 (closed).