Add unit test for reading problem from dataset.

Add four unit tests, which are
* public void Should_be_able_to_read_valid_dataset() {...}
* public void Should_be_able_to_detect_null_dataset() {...}
* public void Should_be_able_to_detect_void_dataset() {...}
* public void Should_be_able_to_detect_invalid_feature_count() {...}