Basic multi png graph display working for Zones #197
Merge remote-tracking branch 'origin/master'
Consistent Y scaling for Zone graphs.  Closes #197
Fixed 2 old tests where the logic was wrong. They were assuming the simulator could exit early because pending delayed airborne exposures would not create new infections. But they were making that assumption by looking at the state (immune) of the destination unit _on the day the exposure was generated_ and assuming that state would still be in effect _on the day the exposure actually happens_.
Eliminated a redundant event type, the "Attempt to Infect" event. Watching for Exposure events with the flag adequate=True set can take the place of Attempt to Infect events.
Merge branch 'master' into python-tests
added tests for probability at form level. fixed issue with modal not properly displaying a validation error when saving. Refs #190.