Allow VERSION.data to be optional to enable logical deletion

Changes proposed: - make VERSION.data 0..1 This change allows logical deletion to be supported, while retaining the integrity of the repository of VERSIONED_OBJECTs (such as an EHR), so that previous views still show the content. Due to this change, content could be deleted from other versions as well, including earlier versions and branch versions. The change does not force deletion to only be the most recent trunk version. This allows for re-instatement of content (when accidental deletion has to be rectified), but might also allow deletions when they should not occur. It is up to application software and higher level components to implement such checks. In theory, the lifecycle_state attribute should be set to the code for "deleted" if the content is deleted, but we do not force this.