SYSML21 — Port::subports should subset subobjects

The Systems Library feature `Ports::Port::subports` is a composite feature typed by the port definition `Port`, which is a kind of `Object`, which is a structure. Therefore, `checkFeatureSubobjectSpecialization` requires that it have an implied subsetting of `Objects::Object::subobjects`. However, in a library model, it is better practice to have this subsetting be explicit rather than implicit.