This type represents an ENTITY.  An entity is any modelling item.


An entity may be of one of the types:

 - PK_ATTDEF_t: an attribute definition
 - PK_ATTRIB_t: an attribute
 - PK_GROUP_t:  a group
 - PK_TRANSF_t: a transform
 - PK_TOPOL_t:  any topological entity
 - PK_GEOM_t:   any geometrical entity