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   Computes the principal axes for an inertia matrix. 
   Input u is a set of vectors for which the routine computes angles between 
   the axes in the base and axes in the principal frame.
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   Form:
   [inertiaP, angle] = PrincipalAxes( inertia, axis )
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   ------
   Inputs
   ------
   inertia   (3,3)   The inertia matrix
             (1,6)
   u         (3,n)   Vectors in the base frame

   -------
   Outputs
   -------
   inertiaP  (3,3)   The inertia matrix
   angle     (1,n)   Angle between u in base and u in principal axes (deg)
   error     (3,3)   Error

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