Write a Python script which computes angular diameter distance between two objects
at redshifts z1 and z2 in the case of a flat cosmological model (Omega_k = 0) with
H_0 = 71/km/s/Mpc and Omega_M = 0.27, and use it to reconsider the exercise with
an alien astronomer by calculating all three angular diameter distances: DA(0,za)
- between you and an alien at redshift za = 1, DA(0,zq) - between you and a quasar
at zq = 2 and DA(za,zq) - between alien and quasar.
