Use version suffix with omexManifest/content@format

[jonrkarr]

My understanding is that the value of omexManifest/content@format should continue to be http://identifiers.org/combine.specifications/omex-metadata. Since the details of the new version diverge from that of the previous version, I think it would be helpful to use version suffixes to indicate which version is being used.

[CiaranWelsh]

I'm not sure exactly how to resolve this - @jhgennari @nickerso perhaps you have some input here?

[jonrkarr]

Instead of http://identifiers.org/combine.specifications/omex-metadata, manifests could be encouraged to use http://identifiers.org/combine.specifications/omex-metadata.version-1_2.