restricted : Result instance
The restricted model is assumed to be nested in the current
model. The result instance of the restricted model is required to
have two attributes, residual sum of squares, ssr, residual
degrees of freedom, df_resid.
demean : bool
Flag indicating whether the demean the scores based on the residuals
from the restricted model. If True, the covariance of the scores
are used and the LM test is identical to the large sample version
of the LR test.