xsd-fu: Generalise language type mapping (3)

- Remove unused helper functions from templates
- Move used helper functions from Pojo.template into fu.py object properties for direct template use
- Model object superclass is now defined directly as a property
- Unnecessary and unused method and ctors removed from abstract AbstractOMEModelObject

Note that this patch *does* introduce some changes into the generated ome-xml model code.  Model objects derived from AbstractOMEModelObject no longer chain up to superclass ctors (which have been removed).