CWM11 — Optimize Instance data values

The Instances (hence MultiDimensional) metamodel is very wasteful
in requiring a separate DataValue object for each simple string slot value:
this in effect doubles the number of objects for value-oriented schemas such
as Dimension definitions (in MultiDimensional where DataValue is inherited
into MemberValue). This is a problem for XMI files and their processing, but
even more so for repository implementations which tend to have management
overheads associated with each object. Moreover these DataValue objects end
up being directly contained in the MemberSet for the Dimension, which surely
was not the intention. This means that when navigating from the Dimension to
process its Members one also has to filter out a large number of these
unwanted Data/MemberValue objects.

see below