Allow component graph to be a DAG instead of a tree

Certain scenarios, like a battery connected to 2 inverters are not
a tree graph, but still a directed acyclic graph which we allow.

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/sdk/microgrid/_graph.py

@@ -761,7 +761,7 @@ def _validate_graph(self) -> None:
         if self._graph.number_of_edges() == 0:
             raise InvalidGraphError("No connections in component graph!")
 
-        if not nx.is_tree(self._graph):
+        if not nx.is_directed_acyclic_graph(self._graph):
             raise InvalidGraphError("Component graph is not a tree!")
 
         # node[0] is required by the graph definition