The [3][3] element of the 3x3 matrix used to transform Cartesian
coordinates in the ATOM_SITE category to fractional coordinates
in the same category. The axial alignments of this
transformation are described in _atom_sites.Cartn_transform_axes.
The 3x1 translation is defined in
_atom_sites.fract_transf_vector[].
|x'| |11 12 13| |x| |1|
|y'|fractional = |21 22 23| |y|Cartesian + |2|
|z'| |31 32 33| |z| |3|