Saving epidemic data in files

[odunbar]

We have the ability now to run an epidemic and save it within an object using epidemic_data_storage.py. So why not save to file and so we do not need to run epidemics for every data assimilation example .

It would be good to have a preferred format for file type. I mention pickle as an example because we want to store a class. But perhaps it's possible to do this type of thing with JSON or something too.