Whether a cells can have None as its value.
This is a property of Model, Space and Cells. If
allow_noneof a cells is False, the cells cannot have None as its value. Assigning None to the cells or its formula returning None raises an Error. If True, the cells can have None as their value. If set to None,
allow_noneof its parent is looked up, and the search continues until True or False is found.
True if the cells can have None, False if it cannot, or None if a default value from the parent is to be used.