add_meta should error if abbreviation is not defined in namespaces

[cboettig]

No description provided.

[hlapp]

Should it? It might warn, but maybe an error condition is only reached when there is still an undeclared namespace at the time of writing out the nexml object. And arguably, even then it is only an error if one were to require that the user can't generate a NeXML file that fails to validate (which doesn't mean it would also fail to parse).