Handling of the Guids when exporting a model for a new phase

When a new phase is created for an model, Revits generates new internal Guids in the RVT-Database for the elements.

This results also in new GUID's in the IFC file, after exporting the new phase. This new GUID's break the relation to external databases, that rely on this GUID's as a mapping key.

I propose two things:

- The IFC export could export each selected phase an separate IfcProject in the file and make it possible to export also two or more phases in one file
- The IFC export could write the relation of the elements that are in both phases within the IFC file, so that the consumer of the file can follwow the path of the element and can use the mapping of the GUID's.

The issue names no files, tests, or exporter entry points. Start by locating the IFC export handling for phases and element GUIDs, then determine how phase selection and cross-phase relationships are represented. Done means an agreed, tested behavior for preserving mappings and/or exporting multiple phases with usable relationships.

Written by the indexing model from the issue text.