Use GUIDs for enumeration values in IfcSimplePropertyTemplate

Description of the proposal:

The enum values in `IfcSimplePropertyTemplate` do not have a GUID. I propose to add a GUID for each enum value.

What do we win:
We can identify and migrate changed enum values. We can relate all enum values per values to external sources.

What do we loose
nothing

Schema impact:
low

Instance model impact:
All future model files woul have this additional GUIDs

Backwards compatible:
depend, on how older systems import files, that contain this addition GUIDs

Automatic migration possible:
yes

Additional implications:
unknown

Start by locating the IfcSimplePropertyTemplate schema and its enumeration values. Determine how adding a GUID to each value affects the schema and future instance model files, including backward compatibility and automatic migration; the issue is complete when those implications and the required schema change are resolved.

Written by the indexing model from the issue text.