The new PBS documentary on dancer-choreographer Twyla Tharp is called Twyla Moves. In retrospect, that sounds a bit weak. It really should be called Twyla Moves And Won’t Stop As Long As She Has a Detectable Pulse, a title that might perhaps begin to capture the fierceness with which Tharp, who turns 80 this year, approaches both work and life. It’s a fierceness that led her at one point to take boxing lessons with Teddy Atlas, who trained Mike Tyson, to get in the best possible condition for a piece she was doing. “I eventually had to stop boxing because I got hit and broke my nose, ” she recalled in a recent interview.