Add interface for adding subject identifiers to infrastructure elements

[ajenhl]

As entities can have subject identifiers associated with them via a property assertion, so infrastructure element should be able to have subject identifiers associated with them. Obviously the underlying topic map has this functionality, that just needs to be exposed in the EATS interface.

One open question is whether such subject identifiers are directly associated with an element, or kept at a distance by something akin to an entity property assertion.