METAR: enable reports from multiple stations

METAR: enable reports from multiple stations

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.