Make sourcePublisher and sourceDate optional

so that it is the same across all schematas

Author: egli

resources/schema/recorded_production.rnc

@@ -43,8 +43,8 @@ start =
                      element jahr { xsd:integer }?
                   },
                   element ncc {
-                     element sourcePublisher { xsd:token },
-                     element sourceDate { xsd:token },
+                     element sourcePublisher { xsd:token }?,
+                     element sourceDate { xsd:token }?,
                      element narrator { xsd:token }+,
                      element setInfo { xsd:positiveInteger },
                      element revisionDate { xsd:date }