Entities added after trailing entities deleted

[brunopostle]

ifcmerge relies on the Native IFC principle that no application will reuse the IDs of deleted elements. However there is a theoretical possibility of failure in this situation:

1. Application deletes some entities, one or more of them are the 'last' entity with the highest step-ID in the file.
2. File is saved to disk and opened again by the same or another application.
3. The new application adds new entities with ID values that have previously been used.

ifcmerge will typically refuse to merge such a file as the class of the ID will likely have changed. This is a robust failure mode, and avoids corruption.

It should be possible for ifcmerge to detect this situation and simply renumber the offending IFC file such that there is no clash before merging.