"Deprecate" dataModel

[msdemlei]

As laid out in http://ivoa.net/documents/Notes/TableReg, TAPRegExt's dataModel was not a good idea. We should state that new standards shouldn't use it (and probably use what TableReg says, but then we'd have to think about endorsing TableReg).

[msdemlei]

(By and large) fixed via #11