KERML — Binary association ends always unique

In the Links model library, BinaryLink::source/target are defined as

assoc all BinaryLink specializes Link {
 feature participant: Anything[2] nonunique ordered redefines Link::participant;
 readonly end feature source: Anything[0..*] subsets participant;
 readonly end feature target: Anything[0..*] subsets participant; }

making source/target unique by default (subsets don't inherit ordering/uniqueness). This prevents modelers from defining the corresponding features of associated classifiers as nonunique (see KERML-37), because nonunique is less restrictive than unique.

BinaryLink::source/target nonunique

Add nonunique keyword to BinaryLink::source/target in Links.kerml.