Declare what block will be used in this engine.

        It should be call before adding or setting any component.
        Blocks order will be preserved for runnning task.

def requires(self, *names):
        """ Declare what block will be used in this engine.

        It should be call before adding or setting any component.
        Blocks order will be preserved for runnning task.
        """
        if len(names) == 0:
            raise ValueError("You should give at least one block name")
    
        if self._blocks is not None and len(self._blocks) > 0:
            raise ReliureError("Method 'requires' should be called only once before adding any composant")
        for name in names:
            if name in self._blocks:
                raise ValueError("Duplicate block name %s" % name)
            self._blocks[name] = Block(name)
        self._logger.info(" ** requires ** %s", names)