1573 pathway step save

[fredkingham]

the pre_save work exactly as you suggested.

Also previously there was a bug where if you defined the model on a step e.g.

class SomeStep(Step):
   model = MyModel

then SomeStep.model == None, so I have changed that as well.

There is a pull request ready for rfh when this is merged.

[davidmiller]

Honestly I think the bug with declaring model = on the class would be better split out into it's own PR...

Would likely have merged earlier (at least at the start of these reviews!)

Can we try to remember what the use case for save_data is ?
I was struggling...

[fredkingham]

This is two years old, and I think we've lost the context.