code: def mmf(x, alpha, beta, kappa, delta):
      
      return alpha - (alpha - beta) / (1. + (kappa * x)**delta)
docs: Morgan-Mercer-Flodin
      http://www.pisces-conservation.com/growthhelp/index.html?morgan_mercer_floden.htm
  
      Parameters
      ----------
      x: int
      alpha: float
      beta: float
      kappa: float
      delta: float
  
      Returns
      -------
      float
          alpha - (alpha - beta) / (1. + (kappa * x)**delta)