[Data-modeling] Minor change to opera schema

[Jeff Prucher]

I don't think anyone is really using this schema, but just in case they are,
I'd like to add a CVT between "opera" and "opera character" that will
contain the vocal range of the character; this seems like the most basic
piece of information someone would want to know about an opera character,
but we don't currently capture it.  (I want to make it CVT for cases where
the same character appears in different operas; otherwise we could just hang
voice off of "opera character".)  Obviously, this would break any code using
this schema, but since there's remarkably little data in this property, I'm
guessing it's safe to do.  Let me know if I'm wrong!

Jeff Prucher
Type Librarian & Ontologist
Metaweb Technologies, Inc.