IC623X3:

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   Convert an inertia matrix from a 1x6 to a 3x3 format.
   The original form  is 

   [Ixx Iyy Izz Ixy Ixz Iyz]

   to 

   [Ixx Ixy Ixz
    Ixy Iyy Iyz
    Ixz Iyz Izz]

   Since version 1.
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   Form:
   y = IC623X3( x )
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   ------
   Inputs
   ------
   x           (1,6)     [Ixx Iyy Izz Ixy Ixz Iyz]

   -------
   Outputs
   -------
   y           (3,3)     Inertia matrix

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