[Gzz] Ids

[Benja Fallenstein]

I've converted Gzz to use the urn-5 namespace. All cell and dimension 
ids must be URIs now.

This means we need a way to convert from the old, non-URI id format (of 
the CVS development version, not of Gzz 0.6).

- If the old ID is "home-cell", use:
 <urn:urn-5:uzPa+vB7kMBWMZWw-Og58D2NXGeZ:1>

- If the old ID is "home-id", use:
 <urn:urn-5:uzPa+vB7kMBWMZWw-Og58D2NXGeZ:2>

- If the old ID is "home-block", use:
 <urn:urn-5:uzPa+vB7kMBWMZWw-Og58D2NXGeZ:3>

- Otherwise, use <urn:urn-5:OLDID>, where OLDID is the old id.

The home-* ids cannot be used as <urn:urn-5:home-*>, because that would 
be an abuse of the namespace (uniqueness wouldn't be guaranteed).

-b.