Converging the data models

[Jonathan McDowell]

Thanks to constructive criticism from the CDS gang, I have
done another try at my "toy" model for characterization.

http://hea-www.harvard.edu/~jcm/vo/xsd/char2.xsd
is a self contained schema where everything is defined very
simply, just like the initial CDS version, but which still has
the property that it involves valid instances of STC and Quantity
objects - just not as explicitly as before, but will still
let STC and Quantity parsers and class software work with it.

 - Jonathan