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   Take the cross product a matrix of vectors times a flex transformation matrix.
   Returns one vector per mode. n is the number of nodes and m is the number
   of modes. phi is in blocks of m, i.e. phi[mode1, mode2, ..., modem]
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   Form:
   x = VXPhi( v, phi )
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   ------
   Inputs
   ------
   v                (3,n)   Matrix of vectors
   phi              (3,n*m) Nodal displacements per modal displacement
   

   -------
   Outputs
   -------
   x                (3,m)   Product

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