specification: Add metadata for HDF5 embedding

- add Hdf5Data element for use in Pixels and Mask
- split TiffData out of ome.xsd and allow use in Mask

This is intended for comments, not merging at this time.  The intent isn't to get OME-HDF5 working or fully specified at this point, but to add the necessary bits to the schema to allow OME-XML to reference datasets (pixel data hypervolumes) in an HDF5 file.

--exclude
Wouldn't you conceivably want more than one?