IfcLoop and IfcVertex are not ABSTRACT?

Description of the proposal:
Make `IfcLoop` and `IfcVertex` entities abstract.

They do not possess any attributes, therefore I don't see any reason to see them in a physical file. E.g.:

> #90 = IFCLOOP();

Therefore: make them `ABSTRACT`.

What do we win: clearer schema.

What do we loose: nothing.

Schema impact: backward compatible, I would even suggest updating the old schemas as well.

Instance model impact: No chance to submit a file with entities like `IfcLoop` and/or `IfcVertex`.

Backwards compatible: Yes.

Automatic migration possible: ?

Additional implications: ?