'Returns the Z selector matrix in the observation equation.
Parameters
r : int
In the context of the ARMA model r is max(p,q+1) where p is the
AR order and q is the MA order.
Notes
Currently only returns a 1 x r vector [1,0,0,...0].  Will need to
be generalized when the Kalman Filter becomes more flexible.'

@classmethod
def Z(cls, r):

    arr = zeros((1, r), order='F')
    arr[:, 0] = 1.0
    return arr
