Rename "Remote Repositories", add Identifier and Publication Date to Assets

The identifier allows to provide a short reference to the asset to be used in documents (similar to the Identifier Property used in resources).

The Location property replaces the Remote Repositories to be closer to the resource doctype and allow to also reference local information related to the asset.

Since the asset provides a version we also want to provide a publication date.

Note that also the release doctype also provides version and publication date, we usually need to duplicate this information. The release is a record and therefore should not reference an asset (which is a versioned document).