code: def radiation_values(self, location, timestep=1):
          
          # create sunpath and get altitude at every timestep of the design day
          sp = Sunpath.from_location(location)
          altitudes = []
          dates = self._get_datetimes(timestep)
          for t_date in dates:
              sun = sp.calculate_sun_from_date_time(t_date)
              altitudes.append(sun.altitude)
          dir_norm, diff_horiz = ashrae_clear_sky(
              altitudes, self._month, self._clearness)
          glob_horiz = [dhr + dnr * math.sin(math.radians(alt)) for
                        alt, dnr, dhr in zip(altitudes, dir_norm, diff_horiz)]
          return dir_norm, diff_horiz, glob_horiz
docs: Lists of driect normal, diffuse horiz, and global horiz rad at each timestep.