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   Compute position, velocity and mass derivatives for a launch vehicle.
   The model assumes a rotating spherical planet.
   The model can handle multiple stages and different
   fuels for each stage. The dry masses and exhaust velocities are entered
   using the data structure d. For each stage you add a fuel state to x.
   Thus a 3 stage vehicle will have 9 states and d.mStage, d.thrust,
   d.uE and d.cDA (the drag coefficient) will each have three entries.
   The data structure is the same as is used in LaunchVehicle2D.
   Any planet that is included in AtmDens1 can be simulated.
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   Form:
   xDot = LaunchVehicle3D( x, t, d )
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   Inputs
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   x           (:,1)  [r v massFuel]
   t           (1,1)  Time (not used)
   d           (1,1)  Data structure
                      .mStage (1,:) Dry mass of each stage (kg)
                      .thrust (1,:) Thrust of each stage (N)
                      .uE     (1,:) Exhaust velocity of each stage (km/s)
                      .cDA    (1,:) Drag coefficient of each stage times area
                      .g      (1,1) Gravity (km/s^2)
                      .r      (1,1) Planet radius (km)
                      .planet (1,:) Planet name

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   Outputs
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   xDot        (:,1)  Stage structural ratios

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