add type field automatically to all objects when creating

[alastair]

Sometimes we have lists of objects that are of varying types. For example, an ItemList could contain a DefinedTermSet, an Annotation, and an AudioObject.
At the moment I'm distinguishing these items by adding __typename to the output. However, all objects have a type field which is supposed to be the semantic web type (the same type that we serialise to @type in the json-ld serialiser).
If we automatically filled in this type (+additionalType if the object has more than 1 type) then we could use it for filtering or processing data, but I'm not sure if it makes sense to do it. __typename is pretty stable for this case anyway.