Make pylab import and lower origin explicit in demo notebook
Added zernike.py based analytic optics

  - ZernikeThinLens works like ThinLens but calls poppy.zernike.zernike
    to compute the actual OPD. Also incorporates an explicit pupil_radius
    argument (optional) and improved radius-guessing heuristic when unset
  - ZernikeOptic (may eventually move to wfe.py) takes a pupil_radius and
    coefficients as kwargs. coefficients is a sequence of (n, m, k) tuples
    where n, m are the radial and azimuthal indices of a Zernike polynomial
    and k is a coefficient in meters RMS wavefront error in that Zernike
    component