Nudge vertical scaling factors to observed ground pressure

[FriedrichKlappenbach]

The calculation of the vertical scaling factor is based on the altitude dependent density dens(alt).

Since meteorology in the map-files is a generic standard atmosphere, this rarely matches the true pressure at the instruments altitude.

It should be better to use the pressure dependent density dens(pressure) and start at the point of observed groundpressure at the instrument and perform the integration on over the pressure rather than altitude.

Alternatively: use Instruments altitude above sealevel as starting point.