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   Compute the final mass from the rocket equation.
   You use any combination
   of inputs as vectors that make sense. However, if you want internal
   plots you should only have one be an array. Otherwise it will always
   plot with x being the first input that is > length 1.
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   Form:
   massFinal = RocketEquation( deltaU, massInitial, uExhaust )
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   Inputs
   -------
   deltaU       (1,:)   Required delta-v
   massInitial  (1,:)   Initial mass
   uExhaust     (1,:)   Exhaust velocity

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   Outputs
   -------
   massFinal    (1,:)   Final mass

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