RHSPressure

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   RIght-hand-side for a thruster based on the input pulsewidth.
   t is not used. This is compatible with RK4 and RK45.
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   Form:
   f = RHSUKF( x, t, d )
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   ------
   Inputs
   ------
   x         (1,1)  Position
   t         (1,1)  Time (not used)
   d         (1,1)  Data structure
                    .mDot (1,1) Mass flow
                    .pw   (1,1) pulsewidth
                    .dT   (1,1) Time step

   -------
   Outputs
   -------
   f         (1,1)  State derivative

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