IConvP:

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   Transform a a 3x3 inertia matrix into 6x1 compact inertia vector.

   Vector form:   [Ixx; Iyy; Izz; Ixy; Ixz; Iyz]

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   Form:
   IVec = IConvP( IMat );
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   ------
   Inputs
   ------
   IMat          (3,3)    Full inertia matrix

   -------
   Outputs
   -------
   IVec          (6,1)    Compact inertia vector

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