CR 1.7 Add MDVersion to MDTYPE and mdRef

Many metadata schemes are versioned, and it may be useful to know the version used when processing the document. This information is already implicitly in the schemaLocation and/or namespace information (for schemas where versioning is relevant), but processors may benefit from an explicit statement of metadata version. This would go along with the MDTYPE attribute and should be optional. For instance, MODS is currently in version 3.2 and about to go to 3.3; PREMIS is in v1.1 and will soon go to 2.0

Requested by Rebecca Guenther, Library of Congress