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   Convert from the ECI frame to the NED frame.
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   Form:
   b = ECIToNED( r, opt, f )
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   ------
   Inputs
   ------
   r           (3,1) Position vector
   opt         (1,:) 'matrix' or 'quaternion'
                       (default is matrix)
   f           (1,1) Flattening factor (optional)

   -------
   Outputs
   -------
   b           (3,3) Transformation matrix
               (4,1) Quaternion

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