#2636 Basic Weather: QNH discontinuity

I remembered an old bug I forgot to made a ticket for.

As QNH changes when flying between two METARs in Basic Weather, at least atmosphere/P-psf and aero/qbar-psf of JSBSim change too fast. The FG properties /environment/pressure-* are also affected. This causes problems e.g. in altitude hold or autothrottle which suddenly find an unrealistically big error and attempt to correct it.

Reproduction:

1. Run $ fgfs --aircraft=UFO with basic weather and live data in airport-populated area and fly a straight line. Watch /environment/pressure-sea-level-inhg change abruptly.
2. Switch weather scenarios. The sea level pressure is interpolated correctly.

This means that whichever is interpolating between weather scenarios is no longer working properly when there is a live METAR change.

[Anonymous]

Definitely related: and old forum report https://forum.flightgear.org/viewtopic.php?f=69&t=16630&start=60#p195737
Could be related: https://sourceforge.net/p/flightgear/codetickets/2485/