Check signature of returned types instead of class

[lisad]

We have some code to help make sure the developer doesn't do something easy, dumb and hard to debug -- to forget to return a value from a step entirely. If we don't check this specifically the errors we get from assuming the step did return a dict (or a DataFrame) are much more random and slower to track down.

    if not "DataFrame" in str(result.__class__):

However, we shouldn't be checking to see if the class name has a string in it - we can instead see if the return value has a "to_dict" method. That could be even more useful.