origin_structures not defined..

[Jintram]

There appear to be origin_structures lacking..

Traceback (most recent call last):
  File "./test_directbinding.py", line 329, in <module>
    s.step()                       
  File "/home/jintram/jintram_jul2012_egfrd/egfrd.py", line 445, in step
    f(self, domain, [domain.domain_id])         # fire the correct method for the class (e.g. process_single_event(self, Single))
  File "/home/jintram/jintram_jul2012_egfrd/egfrd.py", line 1760, in process_single_event
    domains = [self.make_new_domain(single)]
  File "/home/jintram/jintram_jul2012_egfrd/egfrd.py", line 1588, in make_new_domain
    domain = self.try_pair (single, obj)
  File "/home/jintram/jintram_jul2012_egfrd/egfrd.py", line 2329, in try_pair
    assert self.check_domain(pair)
  File "/home/jintram/jintram_jul2012_egfrd/egfrd.py", line 2610, in check_domain
    associated = [domain.origin_structure.id, domain.target_structure.id]
AttributeError: 'MixedPair2D3D' object has no attribute 'origin_structure'

The following classes need to have an origin_structure defined: InteractionSingle, MixedPair2D3D, MixedPair1DCap, CylindricalSurfaceInteraction.