Add type hinting

[SSchotten]

This PR attempts to add more readability/clarity to the code by adding type hints in all function definitions. The main changes are as follows:

• Introduced dataclasses to describe the contents of a DQ rules json file
• Slightly modified validate_and_load_dqrules so json.loads does not have to be called twice in a row
• Renamed parameters with suffix _dict to explicitly indicate a dictionary
• Removed type descriptions from docstrings (as these are now captured by the type hints)
• Slightly modified readme and readme-dev