code: def lai(self):
          
          ndvi = self.ndvi()
          lai = 7.0 * (ndvi ** 3)
          lai = where(lai > 6., 6., lai)
          return lai
docs: Leaf area index (LAI), or the surface area of leaves to surface area ground.
          Trezza and Allen, 2014
          :param ndvi: normalized difference vegetation index [-]
          :return: LAI [-]