make 'a' slider snap to zero if |a| < 0.03, #23
make 'a' slider snap to zero if |a| < 0.03, #23

Signed-off-by: Chris Malley <74b2712074b149d140079dbd7e875955d192e617@pixelzoom.com>

phetsims/graphing-quadratics,phetsims/graphing-quadratics,phetsims/graphing-quadratics