A vocable is one item that is part of the OpenChoice and ClosedChoice rules.
It consists of an XMP value (that can be any Object and has to be interpreted
by the client) and a label for the UI.
Currently Vocables can be of the types
XMP Type --> Java Type
Text --> String
Integer --> Long
Real --> Double
Boolean --> Boolean
Date --> ISO8601 String
TODO: Fixed Array of Integers for some Exif special cases