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   The right hand side of a 2D launch model.
   The states are vertical and horizontal position, velocity
   and flight path angle. The model assumes a flat Earth that 
   isn't rotating. 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 7 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 LaunchVehicle3D.
   Any planet that is included in AtmDens1 can be simulated.
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   Form:
   xDot = LaunchVehicle2D( x, t, d )
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   Inputs
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   x           (:,1)  [x h v gamma 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
   -------
   xDot        (:,1)  Stage structural ratios

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