N’DJAMENA, Nov 19 (Reuters) – The United Nations is pushing to gain access to al-Fashir, the famine-stricken city in Darfur where witnesses have reported mass reprisals since a takeover by Sudan’s Rapid Support Forces last month, U.N. aid chief Tom Fletcher said. Fletcher told Reuters it would be a huge task to provide aid to the city, which would be treated as a “crime scene” for investigations following reports of systematic executions, detentions, and rapes. Many of those thought to have