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   Aircraft engine model. Uses a piecewise linear model to map throttle
   to commanded power and a lag to model the response of the throttle
   to the input command.
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   Form:
   g = ACEngine( x, d, control, altitude, machNo )
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   ------
   Inputs
   ------
   x                     State
   d                     Engine data
   control               Controls
   machNo         (1,1)  Mach number
   altitude       (1,1)  Altitude

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   Outputs
   -------
   g                     Output data structure
                         .torque      (3,1) Core torques
                         .force       (3,1) Core forces
                         .xDot        (:,1) Engine state derivatives
                         .mDot        (1,1) Fuel consumption
                         .inertiaDot  (3,3) Inertia derivative due to fuel consumption
                         .cGDot       (3,1) CG derivative due to fuel consumption
                         .torqueRotor (:,1) Torques on rotors

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