datatypeNamespace => datatypeLibrary?

In writing up the changes we decided on for the tutorial, it struck me that
it might be less confusing if the "datatypeNamespace" attribute had a name
that didn't include "namespace", such as "datatypeLibrary".  Since we are
not using QNames to identify datatypes, there is no need to bring in the
concept of namespaces, with all the baggage and confusion that it usually
brings along with it, for identifying namespaces.  We simply have a URI that
identifies a collection of datatypes, and a name that identifies a member of
the collection.  There's no need for any concept of namespaces.

If we do this, then we don't need to rename the ns attribute to namespace,
or the nsName attribute to namespaceName (though we might want to do that
anyway).

James