Cleaning up models.py.  Commit after some changes to cov_params.  Clarified docstring, changed 'matrix' to 'contrast',  and added some robustness checks to make sure the inputs make sense.
Cleaning up models.py.  Commit after some changes to cov_params.  Clarified docstring, changed 'matrix' to 'contrast',  and added some robustness checks to make sure the inputs make sense.

matthew-brett/draft-statsmodels,matthew-brett/draft-statsmodels