Made sure we actually would write some stuff to the tree if we had hits

Fix mistake introduce in commit #1bb07c7c: the declaration of "CartesianGridXYZ" had been cancelled inadvertently.

Geant4AssemblyVolume: use the same copy ID for the Geant4 Volume as was set in DD4hep geometry construction

    is now set to [-pi,pi] instead of [0,2pi] if phi IDdescriptor field is signed.

the target is now(?) prefixed with Geant4:: otherwise we get a linking error

It doesn't matter because we don't care about the result