node: have multiple parents, belong to multiple volumes

from http://drupal.org/node/135297 :
> I was hoping the volume outline would allow me to have multple parents / belong to multiple volumes, but this didn't seem to be the case.

I almost added this feature, and the code can be easily adapted to do so. But I didn't do it, because there are some useability difficulties in allowing this. Those difficulties must be thoroughly reviewed and a solution found before we can add such a feature.