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   Compute body velocity from alpha, beta and vT.
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   Form:
   vB = VTToVB( vT, alpha, beta )
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   Inputs
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   vT          (1,1) Velocity
   alpha       (1,1) Angle of attack (rad)
   beta        (1,1) Sideslip angle (rad)

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   Outputs
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   vB          (3,1) Velocity in the body frame

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