fix error

Traceback (most recent call last):
  File "/usr/local/lib/python2.7/dist-packages/pycado/parse.py", line 192, in display_file
    o.build()
  File "/usr/local/lib/python2.7/dist-packages/pycado/pycado_obj.py", line 272, in build
    self.topology = TopoDS().Face(ex.Current())
AttributeError: 'TopoDS' object has no attribute 'Face'