So let’s start with the enticing premise of Luc Besson’s “Lucy,” starring Scarlett Johansson: Human beings only use 10 percent of their brain capacity. Imagine what it would be like if we could access all of it? Well, wow. It would be sort of like ... nothing new. Because, it turns out, in real life, humans pretty much DO use their whole brains. Darn. Now, we could choose to be annoyed that Besson starts with a total myth. Or we could give him a pass — because, hey, the movie is …