OptionalinitializedForecast series (WV) only: when the forecast run was issued — the same for every
point of one run.
OptionaltypeForecast series (WV) only: forecast (Vorhersage) for the nearer points, then
estimate (Abschätzung, a rougher outlook) for the later ones. Not a measured
value either way. Typed open so a value the API adds later still type-checks.
The reading in the timeseries' unit, or null for a point without a value (the
sentinel 99999, mapped by the client). Skip null points before a minimum, maximum
or trend.
One point of a measurements series.