Add forced spring mass oscillator

[mforets]

No description provided.

[mforets]

in this model there is an interesting discussion of the different approaches to model the forcing term, eg.

1- if the analytic form is known, couple the associated ODE to such form and solve a homogeneous problem
2- non-deterministic inputs at sampling times
3- interpolated inputs between sampling times
4- no bloating of the inputs
5- constant input at sampling times coupled to the system, possible the same as 4

cc: @dfcaporale