code: def set_fitness(self, v):
          
          base = self._base
          self.fitness(v)
          if not np.isfinite(v.fitness):
              self.del_error(v)
              return False
          if base._tr_fraction < 1:
              self.fitness_vs(v)
              if not np.isfinite(v.fitness_vs):
                  self.del_error(v)
                  return False
          self.del_error(v)
          return True
docs: Set the fitness to a new node.
          Returns false in case fitness is not finite