Fifty years into a legendary career, most bands are content to rest on their hits, releasing bland blues covers albums or bored-yet-pleasant regurgitations of their earliest work. Procol Harum is not “most bands.”On the 50th anniversary of their debut album, the English prog rock icons continue to remain a singular force in modern rock music, embracing the new-song, best-song philosophy on their first album in 14 years, Novum, out on Friday, April 21, via Eagle Records.