DoubleIntegratorWithAccel:

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   Finds the plant for a double integrator with an acceleration state.

   State vector is [x;v;a]. Input vector is [0;0;da/dt]

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   Form:
   [a, b] = DoubleIntegratorWithAccel( dT )
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   Inputs
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   dT       (1,1)  Time step

   -------
   Outputs
   -------
   a        (3,3)  State transition matrix
   b        (3,1)  Input matrix

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