function pathAnnulus(r0, r1, a0, a1, cx, cy) {
    return _path(r0, r1, a0, a1, cx, cy, 1);
}
path an annulus

@param {number} r0 : first radial coordinate
@param {number} r1 : second radial coordinate
@param {number} a0 : first angular coordinate in *radians*
@param {number} a1 : second angular coordinate in *radians*
@param {number (optional)} cx : x coordinate of center
@param {number (optional)} cy : y coordinate of center
@return {string} svg path