Extend TestEngine so that it can be used for "single-dataset" engines

[nathanfranklin]

In TestEngine, test_iter_datasets and test_remove_dataset add 5 datasets for testing purposes. Some engines only support a single dataset.

[nathanfranklin]

TestEngine could be given given a method that could be overridden for testing single-dataset engines

something like:

    def number_datasets_used_in_testing(self):
        """ Number of datasets used in testing engine

        """
        return 5

[nathanfranklin]

Closed by #224