Compatibility with Qt6 does not wreck Qt5

Adapted GeoModelVisualization to Qt6 and to the recently released newversions of Coin (4.0.2) and SoQt (1.6.2). A number of classes were suppressed totally, since they are not used (eg those dealing with "2d systems" none of which are used here. This was preferable to making dead code compile.

This work should be tested on a variety of platforms before getting merged to main. The branch can stay open until such time as.