cpp: Add bioformats MetadataRoot interface
cpp: BaseMetadata: Add index_type typedef

This type is used for referencing all indexed properties in the
metadata store.
cpp: BaseMetadata: Add byte_array typedef

Used for passing pixel data.
cpp: xsd-fu no longer needs package specifying
xsd-fu: Generalise metadata store index signatures
xsd-fu: Add C++ metadatastore templates