a better approximation of the average temperature and dewpoint temperature

[edwinkost]

In the shortwave radiation module in lines 777-791, this is a better approximation of the average temperature and dewpoint temperature according to FAO:

    # decide whether the day classifies as a wet day on the basis of the
    # daily precipitation amount
    wet_day = prec_daily > self.prec_limit

    # relative humidity [1]
    rel_hum = relative_humidity
    if rel_hum is None:
        # estimate the relative humidity from dewpoint temperature 
        # - if not defined, using the minimum daily temperature as the dewpoint temperature
        if dew_temperature is None: dew_temperature = temp_min_daily - 2.0
        # - if not defined, using the maximum daily temperature as the average temperature
        if  temp_avg_daily is None: temp_avg_daily  = 0.5 * (temp_min_daily + temp_max_daily)
        rel_hum = estimate_relative_humidity(t = temp_avg_daily , \
                                             tdew = dew_temperature)