Make ObservationModule iterable

[york-stsci]

Currently, in order to use an ObservationModule, it must be created, then nextObservation() must be called, then the observation must be run, and nextObservation() called again until it returns None. Instead, this would add an iter method that includes yield, so that it could be done as:

obm = ObservationModule(obs=obs_dict)
for observation in obm:
observation.add_catalogue(catalogue)
observation.add_error()
observation.finalize()

which is both more pythonic and probably makes more sense.