The parent of this assembly in a hierarchy.
This data item is a pointer to _ihm_struct_assembly.id in the
This data item should point to the assembly id of the immediate
parent in a hierarchy.
By convention, the full assembly (top of hierarchy) is assigned parent id 0 (zero).
In case of assemblies that do not conform to a hierarchy,
_ihm_struct_assembly_details.parent_assembly_id is the same as
_ihm_struct_assembly_details.assembly_id indicating a self-parent.