Return a new reference when iterating over a collection. (#1080)

# Description
Return a new reference when iterating over a collection and ensure that
it is decreased again when the instances goes out of scope (memory
managed by dlite).

## Type of change
- [x] Bug fix & code cleanup
- [ ] New feature
- [ ] Documentation update
- [ ] Test update

bindings/python/dlite-collection.i

@@ -157,11 +157,12 @@ struct _DLiteCollection {
     free($self);
   }
 
+  %newobject next;
   %feature("docstring",
-           "Returns a reference to next matching relation."
+           "Returns a new reference next instance in the collection."
            ) next;
   const struct _DLiteInstance *next(void) {
-    return dlite_collection_next($self->coll, &$self->state);
+    return dlite_collection_next_new($self->coll, &$self->state);
   }
 
   %feature("docstring",

bindings/python/tests/CMakeLists.txt

@@ -5,6 +5,7 @@ set(tests
   test_python_bindings
   test_collection
   test_collection_load
+  test_collection_refcount
   test_entity
   test_factory
   test_misc

bindings/python/tests/test_collection_refcount.py

@@ -0,0 +1,64 @@
+"""Test that the reference counting of instances added to a collection
+is working properly.
+
+The test ensures that the reference count is increased when new
+references are created and decreased when the references goes out of
+scope or are deleted.
+
+"""
+import dlite
+
+
+def add_blob(coll):
+    """Adds two blob instances to the collection."""
+    Blob = dlite.get_instance("http://onto-ns.com/meta/0.1/Blob")
+    blob = Blob([10])
+    blob2 = Blob([2])
+    coll.add("blob", blob)
+    coll.add("blob2", blob2)
+
+
+def use_blob(coll):
+    """Access instance using the Collection.get() method."""
+    blob = coll.get("blob")
+    assert blob._refcount == 3  # coll, local blob, global blob
+
+
+def use_blob2(coll):
+    """Access instances using the Collection.get_instances() method."""
+    blob, blob2 = coll.get_instances()
+    assert blob._refcount == 3  # coll, local blob, global blob
+    assert blob2._refcount == 2  # coll, local blob2
+
+
+coll = dlite.Collection()
+assert coll._refcount == 1
+
+add_blob(coll)
+assert coll._refcount == 1
+
+blob = coll.get("blob")
+assert blob._refcount == 2  # coll, blob
+
+use_blob(coll)
+use_blob(coll)
+assert coll._refcount == 1
+assert blob._refcount == 2  # coll, blob
+
+use_blob2(coll)
+assert coll._refcount == 1
+assert blob._refcount == 2  # coll, blob
+
+use_blob2(coll)
+assert blob._refcount == 2  # coll, blob
+
+
+insts = list(coll.get_instances())
+inst = insts[0]
+assert inst._refcount == 3  # inst, blob, coll
+
+del coll
+assert inst._refcount == 2  # inst, blob
+
+del blob
+assert inst._refcount == 1  # inst