Calculate the crossing times in the case of a "typical" lens (zl = 0.5, zs = 2)
and Einstein Cross (zl = 0.04, zs = 1.695) for a microlens crossing with an
effective velocity of 600 km/s over a source with 100 AU in size. Assume a flat
cosmological model with H0 = 75 km/s/Mpc, Omega_M = 0.3 and Omega_Lambda = 0.7.