#4 ADDATM U11 constraint in p_-3_1_c

[Doug du Boulay]

struck a spot of bother with symmetry constraints on
the special positions 1/3 2/3 1/4 and 2/3 1/3 1/4 in
space group p_-3_1_c

ADDATM decides the U11 value should be constant and 0
rather than 2*U12.
silly really.

in principle you can work around it by hacking the
constraint record in the archive file and thereafter avoiding
to run ADDATM again, but that is pretty ugly.

[Doug du Boulay]

example dat file