docs: def monthly(usaf, year, field='GHI (W/m^2)'):
      """monthly insolation"""
      m = []
      lastm = 1
      usafdata = Data(usaf, year)
      t = 0
      for r in usafdata:
          r['GHI (W/m^2)'] = r['Glo Mod (Wh/m^2)']
          r['DHI (W/m^2)'] = r['Dif Mod (Wh/m^2)']
          r['DNI (W/m^2)'] = r['Dir Mod (Wh/m^2)']
          if r['datetime'].month != lastm:
              m.append(t/1000.)
              t = 0
              lastm = r['utc_datetime'].month
          t += float(r[field])
      return m