cpp: ome-xml: 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: ome-xml: Further simplification and reformatting of MetadataConverter
cpp: ome-xml: Align MetadataConvert conditionals with the java equivalent
cpp: ome-xml: Document MetadataConverter
cpp: ome-xml: Add coverage test for MetadataConverter
ome-xml: Add missing LightPathAnnotationRef to MetadataConverter
ome-xml: Add missing InstrumentAnnotationRef to MetadataConverter