feat: norelax option to skip relax recipe steps

[jmbuhr]

No description provided.

[KRiedmiller]

You can just not provide a relax mdp:

                if not hasattr(self.config.changer.coordinates, "md"):
                    logger.warning("Relax task requested but no MD specified for it!")
                    continue

Could be better documented though

[jmbuhr]

true and misleading that currently config.changer.coordinates is marked as required.