xsd-fu: Tighten const correctness in model and store getters
xsd-fu: Store setters for MapPairs must be const references
xsd-fu: MetadataRetrieve getUUID is const correct
formats-api: Add missing GenericExcitationSourceAnnotationRef in MetadataConverter
cpp: ome-xml: Add MetadataConverter
cpp: ome-xml: Generalise MetadataConverter

Use a templated transform function to convert between the retrieve
and store objects using method pointers and indexes.
cpp: Simplify MetadataConverter

Move all functions into a MetadataConverter class.  src and dest
are now class members, so don't require passing around, including
to the transform methods.
cpp: Further simplification and reformatting of MetadataConverter