code: def _lderiv(self,l,n):
          
          return 0.5/nu.sqrt(l)/(nu.sqrt(l)+nu.sqrt(n))**2
docs: NAME:
              _lderiv
          PURPOSE:
              evaluate the derivative w.r.t. lambda for this potential
          INPUT:
              l - prolate spheroidal coordinate lambda
              n - prolate spheroidal coordinate nu
          OUTPUT:
              derivative w.r.t. lambda
          HISTORY:
              2015-02-15 - Written - Trick (MPIA)