SYSML2_ — Cross features for connection definitions and connection usages

Resolution KERML_-87 to issue KERML_-18, which has been approved by the KerML FTF, adds a new `CrossSubsetting` relationship and introduces the concept of cross features for end features of associations and connectors. These changes in KerML require corresponding updates to the concrete syntax, abstract syntax and semantics of connections and connection definitions in SysML, which are based on KerML associations and connectors.