code: def janoschek(x, a, beta, k, delta):
      
      return a - (a - beta) * np.exp(-k*x**delta)
docs: http://www.pisces-conservation.com/growthhelp/janoschek.htm
      
      Parameters
      ----------
      x: int
      a: float
      beta: float
      k: float
      delta: float
  
      Returns
      -------
      float
          a - (a - beta) * np.exp(-k*x**delta)