title: MERGE instead of CREATE for apoc.create.relationship()
body: <p>The procedure apoc.create.relationship(n1,RelType,{},n2) behaves accordingly to the CREATE statement. 
  If a relationship of type <code>RelType</code> already exists between nodes n1 and n2 apoc.create.relationship() will create a duplicate.</p>
  
  <p>Is there an equivalent version of  apoc.create.relationship() that behaves like MERGE?</p>