use GUIDs for relations

[berlotti]

1. to be able to refer to objects outside the file
2. enhance the incremental updates when using IFC in (object versioned) database

• during serialization and reading it is possible to create object first and relations after that
• Valid according to the 2016 version of STEP
• removes dependency on STEP IDs (line numbers)

Follow up: figure out how it works in STEP 2016 version