Fix: a separated atom from a dupliverts structure was still hidden (not visible)

When separating an atom (in the form of, e.g., a sphere) from a dupliverts structure,
where the representative object (e.g., a sphere) is always hidden (not visible), the
separated atom object was still hidden (not visible). Fix: in any case, the 'hide_set'
option of an object is put onto: hide_set(False)