add overrideable json_to_cicero_object method

This addition should allow users to use this library for sending requests to and receiving data from the Cicero API, but also implement their own processing/parsing of the data if they prefer.

The new json_to_cicero_object method in CiceroRestABC() takes JSON from the API and returns it wrapped in a RootCiceroObject() from our cicero_response_classes.py file. This new method is used in _submit_request(), and thus every method in the CiceroRestConnection() class.

Users who prefer API responses NOT to be wrapped by the RootCiceroObject class or any others, may subclass CiceroRestConnection and override this json_to_cicero_object method to perform their own parsing.