Fix LoadComposableNodes action so that loading happens asynchronously (…

…#113)

Fixes #107.

Before, the first call to _load_in_sequence was not asynchronous, unlike it's recursive calls.
This change makes the first call asynchronous.

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

launch_ros/launch_ros/actions/load_composable_nodes.py

@@ -167,5 +167,9 @@ def execute(
                 self.__target_container.node_name
             )
         )
-        # Assume user has configured `LoadComposableNodes` to happen after container action
-        self._load_in_sequence(self.__composable_node_descriptions, context)
+
+        context.add_completion_future(
+            context.asyncio_loop.run_in_executor(
+                None, self._load_in_sequence, self.__composable_node_descriptions, context
+            )
+        )