OscillatorRHS:

--------------------------------------------------------------------------
   Right hand side for a spring.

   Since version 10.
--------------------------------------------------------------------------
   Form:
   xDot = OscillatorRHS( x, t, u, mass )
--------------------------------------------------------------------------

   ------
   Inputs
   ------
   x	      (2,1)  [position;velocity]
   t         (1,1)  Time
   u         (1,1)  Control
   mass      (1,1)  Mass
   omega     (1,1)  Natural frequency

   -------
   Outputs
   -------
   xDot      (2,1)  d[position;velocity]/dt

--------------------------------------------------------------------------