adds a non-persistent log entry to the tree node

def add_log_entry(self, process_name, timeperiod, msg):
        """ adds a non-persistent log entry to the tree node """
        tree = self.get_tree(process_name)
        node = tree.get_node(process_name, timeperiod)
        node.add_log_entry([datetime.utcnow().strftime('%Y-%m-%d %H:%M:%S'), msg])