Schema needs tightening

[martindholmes]

The TEI customization used to validate the XML is fairly rudimentary, with no constraints on some attribute values, default content models, and far more attributes available than we need. Once we're confident that the accurately TEI represents the source data, this should be tightened up.

[martindholmes]

Attribute removal is basically done; there is some customization of valLists to do.