Description of columns in PosGradTensor.txt
Position gradient tensor F:

    F11  F12
F = 
    F21  F22

Column 1.: node ID
Column 2.: node new x (not repositioned in new model box)
Column 3.: node new y (not repositioned in new model box)
Column 4.: dX
Column 5.: dY
Column 6.: F11
Column 7.: F12
Column 8.: F21
Column 9.: F22
