Changed implementation of IDs.

- UG's id was overwritten in vertices because it was used by leaf index
- IDs are now unique over all codims and levels by ORing codim
- Parallel case not tested
- Still: IDs are only implemented for codims 0 and dim. Edges and faces can be done
fairly easily but are not required for P1 elements :-)

[[Imported from SVN: r2986]]