RHS1DWithAccel:

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   Models a double integrator and a bias.
   The inputs are acceleration and the 1 sigma noise of the
   bias. The bias is modeled as a random walk. t is not used.
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   Form:
   xDot = RHS1DWithAccel( x, t, d )
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   Inputs
   ------
   x         (3,1)  State [position;velocity;bias]
   t         (1,1)  Time (not used)
   d         (1,1)  
                    .sigma   (1,1) Noise standard deviation
                    .a       (1,1) Acceleration

   -------
   Outputs
   -------
   xDot      (3,1)  State d[position;velocity;bias]/dt

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