Overwrite is_multiloader for multiloaderwrapper to prevent double nested...

Overwrite is_multiloader for multiloaderwrapper to prevent double nested load() method for single-loader instances

src/dbispipeline/dataloaders/wrappers.py

@@ -37,6 +37,11 @@ class MultiLoaderWrapper(Loader):
         """Returns how many runs this loader will produce."""
         return sum([loader.run_count for loader in self.loaders])
 
+    @property
+    def is_multiloader(self):
+        """Prevent bug if this wrapper only has one loader as element."""
+        return True
+
     @property
     def configuration(self):
         """DB-Representation of this loader."""