The loglikelihood of an AR(p) process
params : array
llf : float
Contains constant term. If the model is fit by OLS then this returns the conditonal maximum likelihood.
If it is fit by MLE then the (exact) unconditional maximum likelihood is returned.
is a (p x 1) vector with each element equal to the mean of the AR process and is the (p x p) variance-covariance matrix of the first p observations.