Expand a concept or collection to all it's narrower concepts.

Expand a concept or collection to all it's narrower concepts. This method should recurse and also return narrower concepts of narrower concepts. If the id passed belongs to a :class:skosprovider.skos.Concept, the id of the concept itself should be include in the return value. If the id passed belongs to a :class:skosprovider.skos.Collection, the id of the collection itself must not be present in the return value In this case the return value includes all the member concepts and their narrower concepts. Returns A list of id's or :class:HTTPNotFound if the concept or collection doesn't exist.