fit null model

def fitNull(self, init_method='emp_cov'):
        """ fit null model """
        self.null = self.mtSet1.fitNull(cache=False, factr=self.factr, init_method=init_method)
        self.null['NLL'] = self.null['NLL0']
        self.mtSet2.null = copy.copy(self.null)
        return self.null