'Parameters
params : tuple, (ar, ma)
try to keep the params conversion in loglike
copied from generate_gjrgarch
needs to be extracted to separate function'

def geth(self, params):

    (ar, ma, mu) = params
    etax = (self._etax + mu)
    icetax = self._icetax
    nobs = etax.shape[0]
    zi = signal.lfiltic(ma, ar, icetax)
    h = signal.lfilter(ma, ar, etax, zi=zi)[0]
    return h
