code: def eta_from_seebeck(seeb,Lambda):
      
      from scipy.optimize import fsolve
      out = fsolve(lambda x: (seebeck_spb(x,Lambda) - abs(seeb)) ** 2, 1.,full_output=True)
      return out[0][0]
docs: It takes a value of seebeck and adjusts the analytic seebeck until it's equal
          Returns: eta where the two seebeck coefficients are equal 
          (reduced chemical potential)