'covariance of parameters based on HJJH
dot product of Hessian, Jacobian, Jacobian, Hessian of likelihood
name should be covhjh'

@cache_readonly
def covjhj(self):

    jacv = self.score_obsv
    hessv = self.hessv
    hessinv = np.linalg.inv(hessv)
    return np.dot(hessinv, np.dot(np.dot(jacv.T, jacv), hessinv))
