use F test to test whether restricted model is correct
Parameters :
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.
Returns :
f_value : float
test statistic, F distributed
p_value : float
p-value of the test statistic
df_diff : int
degrees of freedom of the restriction, i.e. difference in df between models