[Map Function] add assert statement if map function does not return dict or None
IMO, if a function is provided that is not a print statement (-> returns variable of type `None`) or a function that updates the datasets (-> returns variable of type `dict`), then a `TypeError` should be raised. 

Not sure whether you had cases in mind where the user should do something else @thomwolf , but I think a lot of silent errors can be avoided with this assert statement.
Also added to an assert statement that if a dict is returned by function, all values of `dicts` are `lists` Wait to merge until `make style` is set in place. Updated the assert statements. Played around with multiple cases and it should be good now IMO. 