Add support for Linear Time Invariant StatefulSysModel

[juan-g-bonilla]

An LTI system is described through an input vector u, output vector y, and a state vector x. The evolution of the system is given by:

dx = A*x + B*u
y = C*x + D*u

where A, B, C, and D are constant matrices.

This form is very useful to represent a number of engineering models. For example, many control schemes can be represented as a (set of) LTI systems. An LTI system can also be used, for example, to represent delays on a system or introduce continuity to discrete signals (by using overdamped second-order LTI systems).

We could add an abstract LTI system class.