Girlawhirl wouldn't think of playing tennis or taking an aerobics class without wearing a sports bra, but when it comes to yoga, Pilates or even a spinning class, she loves the freedom of a tank with a built in bra. There's something about each of these options from athleta.com that make them favorites – whether it's a pretty print or antimicrobial fabric, or in the case of the Shanti Cami, it's cut a little bit higher so she's never uncomfortable in downward dog…