Fix core state and metadata validation consistency

Author: FraLotito

hypergraphx/core/directed.py

@@ -135,14 +135,7 @@ def add_node(self, node, metadata=None):
         -------
         None
         """
-        if metadata is None:
-            metadata = {}
-        if node not in self._adj_source:
-            self._adj_source[node] = []
-            self._adj_target[node] = []
-            self._node_metadata[node] = {}
-        if self._node_metadata[node] == {}:
-            self._node_metadata[node] = metadata
+        super().add_node(node, metadata=metadata)
 
     def add_nodes(self, node_list: list, metadata=None):
         """

hypergraphx/core/multiplex.py

@@ -564,6 +564,10 @@ def remove_attr_from_edge_metadata_key(self, edge_key, field):
         edge_key = self._normalize_edge(edge_key, layer=None)
         super().remove_attr_from_edge_metadata(edge_key, field)
 
+    def clear(self):
+        super().clear()
+        self._existing_layers.clear()
+
     def __repr__(self):
         return "{}(nodes={}, edges={}, layers={}, weighted={})".format(
             self._type_name(),