Pull NWS Forecast Reference Evapotranspiration (FRET) from the NDFD XML client for irrigation scheduling

Steps

1. Recognize that FRET is not available through the modern api.weather.gov REST API; it's only exposed via the older NDFD XML/GRIB product, at the de facto client endpoint https://digital.weather.gov/xml/sample_products/browser_interface/ndfdXMLclient.php (this domain replaced the legacy digital.mdl.nws.noaa.gov host).
2. Request time-series FRET with query params product=time-series, evp24=evp24 (the "Daily Reference Crop Evapotranspiration" element), begin/end as ISO8601 UTC timestamps, and either a single lat,lon or listLatLon=lat1,lon1+lat2,lon2+... for multiple points in one call.
3. Set Unit=m for metric (or leave default for US units) and XMLformat=DWML to get the standard Digital Weather Markup Language response.
4. Always check for an <error> element inside the 200 OK response body — the NDFD client does not use HTTP status codes to signal query problems such as an unsupported lat/lon or bad date range.
5. Set a generous client timeout (30s+) for multi-point queries and a descriptive User-Agent header identifying your application and a contact email, matching general NWS API etiquette even though this endpoint predates api.weather.gov's formal requirement.