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   Converts flight path angle, velocity and r to orbital elements.

   Type LaunchParamToEl for an example of injection into a circular orbit.
   mu is optional and 3.98600436e5 will be used if mu is not entered.
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   Form:
   [a, e, nu] = LaunchParamToEl( v, gamma, r, mu )
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   ------
   Inputs
   ------
   v           (1,1)  Velocity
   gamma       (1,1)  Flight path angle (rad)
   r           (1,1)  Radius
   mu          (1,1)  Gravitational parameter

   -------
   Outputs
   -------
   a           (1,1)  Semi-major oxis
   e           (1,1)  Eccentricity
   nu          (1,1)  True anomaly

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