This has been a huge year for national milestones: The United States turned 250, Colorado celebrates its 150th and both Route 66 and the Blues Highway are 100 years old. Right alongside it all, Martha Graham Dance Company, the oldest dance company in the nation, revels in its 100-year anniversary. It helps open the Vail Dance Festival July 31 and then presents a full evening of works Aug. 1. It’s quite fitting that the company performs at Gerald…