Merge pull request #854 from rleigh-dundee/cpp-omexmlmetadata

cpp: Add OMEXMLMetadata template
xsd-fu: Generalise index_string to add base type information
xsd-fu: Removed unused argument from MetadataRetrieve getter
xsd-fu: AggregateMetadata contains no getDelegates() method
xsd-fu: Dummy Metadata does not generate _BackReference setters
xsd-fu: Optimise Metadata includes
xsd-fu: Add doxygen comments to MetadataStore and MetadataRetrieve
xsd-fu: Add doxygen comments to AggregateMetadata
xsd-fu: Add doxygen comments to DummyMetadata
xsd-fu: Add doxygen comments to FilterMetadata
xsd-fu: Add doxygen comments to OMEXMLMetadata
xsd-fu: Add doxygen comments to OMEXMLModelObject
cpp: Add doxygen comments to BaseMetadata
cpp: bioformats: Correct and add doxygen comments