Likelihood ratio test to test whether restricted model is correct
Parameters:  restricted : Result instance
large_sample : bool


Returns:  lr_stat : float
p_value : float
df_diff : int

Notes
The exact likelihood ratio is valid for homoskedastic data, and is defined as
where is the likelihood of the model. With distributed as chisquare with df equal to difference in number of parameters or equivalently difference in residual degrees of freedom.
The large sample version of the likelihood ratio is defined as
where
is the average score of the model evaluated using the residuals from null model and the regressors from the alternative model and is the covariance of the scores, . The covariance of the scores is estimated using the same estimator as in the alternative model.
This test compares the loglikelihood of the two models. This may not be a valid test, if there is unspecified heteroscedasticity or correlation. This method will issue a warning if this is detected but still return the results without taking unspecified heteroscedasticity or correlation into account.
This test compares the loglikelihood of the two models. This may not be a valid test, if there is unspecified heteroscedasticity or correlation. This method will issue a warning if this is detected but still return the results without taking unspecified heteroscedasticity or correlation into account.
is the average score of the model evaluated using the residuals from null model and the regressors from the alternative model and is the covariance of the scores, . The covariance of the scores is estimated using the same estimator as in the alternative model.
TODO: put into separate function, needs tests