Implement a smoothing IIR filter with mirror-symmetric boundary conditions using a cascade of first-order sections. The second section uses a reversed sequence. This implements a system with the following transfer function and mirror-symmetric boundary conditions:
c0 H(z) = --------------------- (1-z1/z) (1 - z1 z)
The resulting signal will have mirror symmetric boundary conditions as well.
input : ndarray
c0, z1 : scalar
precision : :
output : ndarray