Internal function for parameter initialization
        Returns Gaussian perturbation

def _perturbation(self):
        """
        Internal function for parameter initialization
        Returns Gaussian perturbation
        """
        if self.P>1:
            scales = []
            for term_i in range(self.n_randEffs):
                _scales = sp.randn(self.diag[term_i].shape[0])
                if self.jitter[term_i]>0:
                    _scales  = sp.concatenate((_scales,sp.zeros(1)))
                scales.append(_scales)
            scales = sp.concatenate(scales)
        else:
            scales = sp.randn(self.vd.getNumberScales())
        return scales