docs: 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__))