Mag:

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   Given a 3-by-n matrix where each column represents a vector, return a
   row vector of the magnitudes of each column.

   Since version 1.
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   Form:
   m = Mag( u )
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   ------
   Inputs
   ------
   u            (:,:)  Vectors

   -------
   Outputs
   -------
   m            (:)   Corresponding magnitudes

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