The New York-based American Contemporary Ballet company could be seen as just one more start-up troupe in an already-crowded dance scene. But ACB is about more than just traditional dance; it’s about reinventing the way in which dancers dance, and protecting them from injury. The idea that dancers should be able to do their job without hurting themselves may sound basic to outsiders, but inside the ballet world, choreographer Lincoln Jones’s quest for pain-free dance is almost revolutionary.