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
@joshmoore I don't think so, but perhaps I've not seen the use for it.

My take on this is that the Hdf5Data contains a single path to a dataset.  Bear in mind this is for Image/Pixels only (at this point).  This dataset *is* the 5D hypervolume; there's no need to have it split up.  It could even be 6D to include RGB or 9D to additionally support Modulo properly.

We could certainly extend Hdf5Data to support subsetting of a larger hypervolume.  Possibly also aggregation of smaller volumes, though I don't think this is necessarily that useful.

What are the circumstances where we might want more than one?