Test optional datasets can be used in tool evaluation (in test_evaluation.py).
Add test_data_parameters.py which test many random DataToolParameter behaviors. Test various paths to DataToolParameter.to_python - including recently enhanced ability to use optional dataset with 'multiple=True' data parameters. Test filtering on datatypes, implicit conversion options (both existing conversions and new ones) both when building HTML forms and picking intial values for workflows. Test special handling of hidden datasets. Tests picking intial datasets when optional and without repeats when used in subsequent calls.