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   Computes the thrust level of a set of thrusters.
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   Form:
   [thrustLevel, turnedOff] = ThrstLvl( time, pulsewidth, fallConst,...
                                  thrustLevelOld, aFall, aRise, bRise )
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   ------
   Inputs
   ------
   time           (1,1) Time since pulse started
   pulsewidth     (n)   Pulsewidth
   fallConst      (n)   Thruster fall time constant
   thrustLevelOld (n)   Previous thrust level
   aFall          (n)   exp(-timeStep/fallConst)
   aRise          (n)   exp(-timeStep/riseConst)
   bRise          (n)   1 - aRise 

   -------
   Outputs
   -------
   thrustLevel    (n)   Thrust level, between 0 and 1
   turnedOff      (x)   Indices of any thrusters which were turned
                        off during this call. 0 <= x <=n

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