Agence France-Presse: Long famous as one of the world’s most polluted megalopolises, gritty Mexico City is beginning to see the fruits of years of clean-up efforts and to breathe somewhat cleaner air, experts say. “The pollution problem is still there, but it is a fact that we have been fighting pollution for more than 20 years and the improvement is undeniable,” said Gustavo Alanis, with the NGO Mexican Center for Environmental Law. Unusually good air quality in recent weeks has unveiled post …