constrainAsSpaceGroup result needs iteration

[pavoljuhas]

Make constrainAsSpaceGroup take effect immediately. As it is the constraints are applied after iterating the resultant object.

from __future__ import print_function
from diffpy.Structure import Structure, Atom
from diffpy.srfit.structure import struToParameterSet, constrainAsSpaceGroup

stru = Structure([Atom('X')])
stru.lattice.setLatPar(3.52, 3.52, 3.52)
phase = struToParameterSet("nickel", stru)
sgpars = constrainAsSpaceGroup(phase, 'Fm-3m')
print("initial constraints:", phase._formatConstraints())
list(sgpars)
print("after sgpars iteration:", phase._formatConstraints())

outputs

initial constraints: []
after sgpars iteration: ['lattice.b <-- a', 'lattice.c <-- a']