Saved Epithelium and Settings files are not forwards compatible

[lavinrp]

by using pickle to save our epithelium and simulation options we implicitly version lock things.
This can cause crashes or incorrect behavior if the saved epithelium or simulation options are somehow incompatible with the rest of the code.

Example: Cell is given a new property that is used in the collision handler. The cells form an old saved epithelium will not have this property and will explode in the collision handler.