Update GFF class.

- Remove obsolete documentation
- Modify save function to None the internal structure
  since it has to be reloaded.  (Temporary-ish hack)
- Add parent gff member to GffElement in order to know
  the origin if part of the GFF structure is shared.
ResourceManager update.

- Fix incorrect variable names.
- Add creature function to aggregate all creatures
  in the resource managers containers.