Register electrode command.

        Add electrode plugin command to context menu.

def register_electrode_command(self, command, title=None, group=None):
        '''
        Register electrode command.

        Add electrode plugin command to context menu.
        '''
        commands = self.electrode_commands.setdefault(group, OrderedDict())
        if title is None:
            title = (command[:1].upper() + command[1:]).replace('_', ' ')
        commands[command] = title