clean up the binding schema

[pahjbo]

The binding schema has grown somewhat organically as new features have been required - it has lead to

• inconsistencies in style
• possible duplication or inconsistency in meaning

various sub-issues will specify exact refactorings

[pahjbo]

possibly define the binding in VO-DML itself.

[pahjbo]

Could add mappings to

• units
• UCDs

also could reference a "profile" that would define coordinate systems etc that a particular model uses in its values.