By Adriana Barrera MEXICO CITY (Reuters) -Former Mexican President Luis Echeverria, who took office in 1970 promising a democratic opening for the country but oversaw six of the harshest years of a so-called “dirty war” against dissidents, has died aged 100. Mexican President Andres Manuel Lopez Obrador on Twitter confirmed the death on Saturday, expressing his condolences to Echeverria’s family. As an elderly man, Echeverria escaped attempts by Mexican prosecutors to indict him for genocide for his role in two infamous massacres of student protesters in 1968 and 1971 that helped define an era…