Modify the connectors to check if values arrive within specified intervals

• Add an option to the Node class (and its from_dict/from_excel methods) to check whether the node arrives (and changes) regularly during a subscription.
• Add a check to the subscriptions in the connectors classes to verify that values for nodes which specify this option change within the required interval

For example a Node object could specify that its value changes at least every five seconds. Then the connector should check during a subscription, whether new values have arrived for the concerned node within the specified interval. If this is not the case, the connector should try to re-connect to the server and emit a warning.