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   Wind gust model. Generates state space equations or spectral densities.
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   Form:
   g = WindGust( h, u0, sigma, wingSpan, f, w )
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   Inputs
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   h             (:,:)    Altitude (m)
   u0            (1,1)    Aircraft mean speed wrt air mass
   sigma         (3,1)    Standard deviation of the gust velocity
                          sigma.u
                          sigma.v
                          sigma.w
   wingSpan      (1,1)    Wing span
   f                      function to be performed
                          'spectral' spectral densities
                          'state'    state space equations for velocity
                          'accel'    state space equations for acceleration
   w             (1,:)    Frequencies at which to compute power spectral densities

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   Outputs
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   g                      Wind gust output of type statespace

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