'Loglikelihood for arma model for each observation, t-distribute
Notes
The ancillary parameter is assumed to be the last element of
the params vector'

def nloglikeobs(self, params):

    errorsest = self.geterrors(params[:(-2)])
    df = params[(-2)]
    scale = np.abs(params[(-1)])
    llike = ((- stats.t._logpdf((errorsest / scale), df)) + np_log(scale))
    return llike
