'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 = self.nobs
    h = miso_lfilter(ar, ma, etax, useic=self._icetax)[0]
    hneg = (h < 0)
    if hneg.any():
        h = np.abs(h)
    return h
