double check input validation for PrePostNEGD

[drbenvincent]

At the moment we test with _series_has_2_levels. But double check if we instead need to enforce _is_variable_dummy_coded. If so then:

• Update PrePostNEGD._input_validation
• Update corresponding tests in test_input_validation.py to check we get the appropriate exception
• Update the example notebook to clarify the data requirements