Validate the resources defined in the options.

def _validate_resources(self):
        """Validate the resources defined in the options."""
        resources = self.options.resources

        for key in ['num_machines', 'num_mpiprocs_per_machine', 'tot_num_mpiprocs']:
            if key in resources and resources[key] != 1:
                raise exceptions.FeatureNotAvailable(
                    "Cannot set resource '{}' to value '{}' for {}: parallelization is not supported, "
                    "only a value of '1' is accepted.".format(key, resources[key], self.__class__.__name__))