'The psi function for Andrew\'s wave
The analytic derivative of rho
Parameters
z : array-like
1d array
Returns
psi : array
psi(z) = sin(z/a)       for \|z\| <= a*pi
psi(z) = 0              for \|z\| > a*pi'

def psi(self, z):

    a = self.a
    z = np.asarray(z)
    test = self._subset(z)
    return (test * np.sin((z / a)))
