steps should only save their model by default

[fredkingham]

At the moment pathways save the world which has lead to several surprising events.

pseudo code resolution looks like the below.

class Step:
    def pre_save(self, data, raw_data, user, patient, episode):
        return data[self.model.get_api_name()] = self.get_model_data(raw_data)

class Pathway:

    def pre_save(self, data, raw_data, user, patient, episode): pass

    def save(self, raw_data, user, patient, episode):
        data = {}
        for step in self.get_steps():
             data = step.pre_save(data, raw_data, user, patient, episode)

        data = self..pre_save(data, raw_data, user, patient, episode)
        patient.bulk_update(data, user, episode)

If a step does not have a model it should blow up.

We can probably run the has Pathway.remove_unchanged_subrecords function and compare the new to_save dictionary with the old one and make sure that we were not relying on some inference logic and explode if so.