Deformations (i.e. the deformation matrix F) can either refer to
the previous time point (-updated Lagrange formulation) or to
time 0 (-total Lagrange formulation).
In both cases, TOCHNOG composes the deformation
tensor with a polar decomposition into F = R U with
F the deformation matrix, R the rotation matrix and U the stretch matrix.
In a -updated_without_rotation approximation,
the rotation matrix is neglected (this saves CPU time).
See also group_materi_memory.