This type represents any TOPOLogical item.


A topological item is a type of entity (PK_CLASS_topol is a subclass of
PK_CLASS_entity).

A topological item may be of one of the types:

 - PK_PART_t:     a part which may be saved or received
 - PK_INSTANCE_t: an instance which positions and orientates an occurrence of
                  a part
 - PK_REGION_t:   a region which is a connected subset of 3-dimensional space
 - PK_SHELL_t:    a shell which is a connected set of oriented faces
 - PK_FACE_t:     a face which is a bounded connected subset of a surface
 - PK_EDGE_t:     an edge which is a bounded connected subset of a curve
 - PK_FIN_t:      a fin which is the junction between a face and an edge
 - PK_LOOP_t:     a loop which is a closed connected set of fins forming a
                  face boundary.
 - PK_VERTEX_t:   a vertex which is a single position in  3D space