Core: add accessors to attached comp value (#339).

CompositeValue (interface) has new accessor methods:
    - getString(String childTypeUri)
    - getInt(String childTypeUri)
    - getLong(String childTypeUri)
    - getDouble(String childTypeUri)
    - getBoolean(String childTypeUri)
    - getObject(String childTypeUri)
    - getComposite(String childTypeUri)

The respective child topic is loaded lazily.

See ticket 339.
See ticket 411.