Strengthen the requirements for the URL Attribute in edmx:Reference and edmx:AnnotationReference

Change in subchapter 4.4.1 and 4.3.1 of the csdl part 3 specification the sentence "The URL may be backed by a CSDL document describing the referenced model" to "The URL SHOULD be backed by a CSDL document describing the referenced model". If it's NOT backed, it should be well known. REMARK: Consumers can load models they already known anyhow from a different location. Accepted: https://www.oasis-open.org/committees/download.php/48622/odata-meeting-30_on-20130321-minutes.html#odata-266