Make Particle resolvable again, since element and model group terms may be undefined at the point of creation
Make Particle resolvable again, since element and model group terms may be undefined at the point of creation

jonfoster/pyxb2,pabigot/pyxb,jonfoster/pyxb1,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb2,CantemoInternal/pyxb,balanced/PyXB,jonfoster/pyxb2,CantemoInternal/pyxb,balanced/PyXB,jonfoster/pyxb1,balanced/PyXB,pabigot/pyxb,jonfoster/pyxb-upstream-mirror,jonfoster/pyxb-upstream-mirror,CantemoInternal/pyxb