Add metadata to the Volumetric Properties

[jordig100]

Add optional <metadatagroup> in the <property> element in order to supply metadata attached to each property.

Additionally it could be added the <metadatagroup> to the remaining volumetric data types: color, boundary and composite.

[martinweismann]

Let's go for the <metadatagroup> in the <property>.

[martinweismann]

#52
I am not sure of the schema though - at all.

[jordig100]

It looks that a child can't reference an element in the parent schema. It would've work it those utility elements would be in a separated schema. But this is not the case.

I think we'd need to leave this on hold, not adding the metadata to the property by now, unless we find a better solution.
I'd like either having to duplicate the definition of the metadata in the volumetric spec either.