'Transforms params to induce stationarity/invertability.
Reference
Jones(1980)'

def _transparams(self, params):

    (k_ar, k_ma) = (self.k_ar, self.k_ma)
    k = (self.k_exog + self.k_trend)
    newparams = np.zeros_like(params)
    if (k != 0):
        newparams[:k] = params[:k]
    if (k_ar != 0):
        newparams[k:(k + k_ar)] = _ar_transparams(params[k:(k + k_ar)].copy())
    if (k_ma != 0):
        newparams[(k + k_ar):] = _ma_transparams(params[(k + k_ar):].copy())
    return newparams
