Update chainTo to support layer links
Update chainTo to support layer links

jonfoster/pyxb-upstream-mirror,pabigot/pyxb,jonfoster/pyxb2,jonfoster/pyxb-upstream-mirror,pabigot/pyxb,jonfoster/pyxb-upstream-mirror,balanced/PyXB,CantemoInternal/pyxb,jonfoster/pyxb2,jonfoster/pyxb2,balanced/PyXB,balanced/PyXB,CantemoInternal/pyxb,CantemoInternal/pyxb