Each 3D Node has a ZPRS (Position Rotation Scale) class instance included.
cf:
- SceneGraph/ZooNodeGraph.h
- Datas/ZooPRS.h

Each object has thus its own reference system with potential direction and
rotation offsets (as in meshes' description in M3D file - first data line)

M3setObjVec() & M3setObjAng() functions set these offsets.

M3movObj() & M3rotateObj() update these offsets with delta values (using
the original offsets as reference).

M3movObjExt() & M3rotateObjExt() update these offsets with delta values 
(using the original reference system as reference). This is "quite as" 
changing these values in the object's father's refence system.