allow adding markers and authors to exams in collections

generic/mixins.py

@@ -51,6 +51,10 @@ class AuthorMixin():
             return "None"
         return authors
 
+    def add_authors(self, users : List[User]):
+        """Add an author to the object"""
+        self.author.add(*users)
+
     def add_author(self, user: User):
         """Add an author to the object"""
         self.author.add(user)