Renames `value-type` to `range` for `missing` nodes

This is in order to distinguish the range of missing nodes from the value-type of resource nodes. The value-type gives the type of the value and the range is a constraint on both type (time, string...) and kind of value (book, place...)

Moves closer to RDF (see http://www.w3.org/TR/rdf-schema/#ch_range )