Description: 
This example for an ODE deals with a springmasssystem attached to
a wall with the object gliding over a rough surface.
see Ritger/Rose: Differential Equations with Applications, P. 327
The rough surface exerts a force f
opposing the motion which is constant (Coulomb friction):
f(x') = { +r if x'>0
{ r if x'<0
x is the displacement from equilibrium, x' is the velocity.
k is the spring constant, m is the mass.
ODE : m*x'' = k*x + f(x')
The system is solved with a variable step size sover based on the
classical RungeKutta method of order 4. It is an implementation
of a simple solver with step size control written for teaching purposes.
For more details on the solver, use 'help myode4'
