Redefine cellobject type as sequence

This allows for multiple byte runs.

As with DFXML, this schema awaits a 1.1 validator, so the order
ultimately won't be important.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Add id and parent reference elements

<id>'s definition was copied from dfxml.xsd, v1.1.0rfc2.

Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>