docs: def hedonic_value(self, novelty):
          '''Given the agent's desired novelty, how good the novelty value is.
  
          Not used if *desired_novelty*=-1
          '''
          lmax = gaus_pdf(self.desired_novelty, self.desired_novelty, 4)
          pdf = gaus_pdf(novelty, self.desired_novelty, 4)
          return pdf / lmax