'check whether the auto-regressive lag-polynomial is stationary
Returns
isstationary : boolean
*attaches*
areigenvalues : complex array
eigenvalues sorted by absolute value
References
formula taken from NAG manual'

def getisstationary(self, a=None):

    if (not (a is None)):
        a = a
    elif self.isstructured:
        a = (- self.reduceform(self.ar)[1:])
    else:
        a = (- self.ar[1:])
    amat = self.stacksquare(a)
    ev = np.sort(np.linalg.eigvals(amat))[::(-1)]
    self.areigenvalues = ev
    return (np.abs(ev) < 1).all()
