Merge branch 'v0.7.0' of https://github.com/IfcOpenShell/IfcOpenShell …

…into v0.7.0

You can now assign and unassign library references

Minor fix.

Rebuild text annotations to have leaders in the same object and use t…

…ext literals. See IfcOpenShell#1153.

New experimental OBJ2IFC script

Purge old 2D element drawing system, so we don't get double the symbo…

…ls on drawings.

Optimise object placement change checking, so medium sized project ex…

…ports are 5 seconds faster at minimum, but can be much higher since false positives are culled.

Minor fix.

Fix IfcOpenShell#1856. Bug where you lost your existing container or …

…aggregation when you added a new aggregation assembly.

Resolve IfcOpenShell#1860. Brickschema is now optional.