The incredible scope of Felix Sparks’ life makes for a great book, but translating it to the screen proved far more difficult. “The Liberator,” written by best-selling British journalist Alex Kershaw and published to acclaim in 2012, told the story of Sparks, who led the liberation of Dachau during World War II. The ethnically diverse group of soldiers he commanded — including Native Americans, Chicanos and white cowboys — were some of the first Allied forces to see the notorious Nazi concentration camp of Dachau when units of the 3rd Battalion, 157th Infantry Regiment, 45th Infantry Division (Sparks’ ragtag Thunderbird group) arrived at the site in the south of Germany on April 29, 1945. The subtitle of Kershaw’s book — “One World War II Soldier’s 500-Day Odyssey from the Beaches of Sicily to the Gates of Dachau” — hints at the depth and drama that preceded that moment.