This patch enables multiple MetarProperties instances tied to the
property tree. For each node's value of /environment/realwx/metar
one MetarProperties instance is created and tied to the property
tree named there. (See FGDATA/Environment/environment.xml for details)
At least one instance will be created to provice backward compatibility
to the existing live-data weather system. This instance (tied to
/environment/metar) fetches a METAR for the nearest airport at a regular
schedule. All other instances fetch a report for airports named in the
property station-id. It re-reads the report every 15 minutes, the
remaining time until the next fetch will be scheduled is in the property
time-to-live. This property can be written to, to extend the live of
this report or schedule an immediate reload by setting it's value to zero.
This patch also provides magnetic variation for the station's location.
If "Live data" was disabled in any way in the global-weather dialog
and reenabled later, the next metar fetch occoured only after up
to 15 minutes after the report expired. This patch triggers triggers
an immediate fetch of a metar after enabling live data.
The automatic runway selection code in startup.nas depends
on a valid metar before /sim/signals/nasal-dir-initialized
is fired. If the METAR arrives after that signal, no automatic
runway selection is performed. This patch waits for a METAR
on the first update() loop of the subsystem.
This is the initial commit for a reworked environment controller.
The main intention is to decouple the individual modules like
metar fetch, metar properties, environment interpolation etc.
to make it easier for other weather module developers to modify
environment settings.
As a side effect, the dialogs for weather-scenario, weather-conditions,
clouds and precipitations have been merged into a single dialog
