Enhance the State Space models paragraph [modeling]

[MSchrau]

• eq 3.9 looks to be out of place. You are covering an introduction to State Space equations. Please consider removing this to complicate example.
• Include a simple example that shows how to make a SS-model of a seriously simple set of Differential Equations (DE)

[MSchrau]

• Make clear SS-equations are not unique from the start and give an example.

[Richard Murray]

Added a simpler example before the more complicated one in Section 3.2.