Fix bug in `StructureBase` class.
Fix bug in `StructureBase` class.

Fixing corner case when element1/element2 attributes
were assigned to new values, but only the
first two elements in the unit cell basis atoms list
were updated.

scikit-nano/scikit-nano,scikit-nano/scikit-nano,androomerrill/scikit-nano,androomerrill/scikit-nano