Work out nearest possible site to lat & lon coordinates
        and return its forecast data for the given time step.
        
        lat:                        float or int.  Latitude.
        lon:                        float or int.  Longitude.
        step:
            metoffer.DAILY          Returns daily forecasts
            metoffer.THREE_HOURLY   Returns forecast for every three hours

def nearest_loc_forecast(self, lat, lon, step):
        """
        Work out nearest possible site to lat & lon coordinates
        and return its forecast data for the given time step.
        
        lat:                        float or int.  Latitude.
        lon:                        float or int.  Longitude.
        step:
            metoffer.DAILY          Returns daily forecasts
            metoffer.THREE_HOURLY   Returns forecast for every three hours
        """
        sitelist = self.loc_forecast(SITELIST, step)
        sites = parse_sitelist(sitelist)
        site = get_nearest_site(sites, lat, lon)
        return self.loc_forecast(site, step)