AP214: Enums in select types mixed with reference types

[ConorStokes]

AP214 has select types (type unions) that mix enums and reference types.

For example rotation_range_measure:

TYPE rotational_range_measure = SELECT 
  (plane_angle_measure, unlimited_range);
END_TYPE; (*-- 10303-105: kinematic_structure_schema*)

Here unlimited_range is an enum and plane_angle_measure is a reference to another STEP entity. The current code-gen doesn't handle this case because IFC encapsulates these in a value type.

[pablo-mayrgundter]

#119
https://github.com/bldrs-ai/IFC-gen/pull/14