Get geoid separation (undulation) from external GNSS

[wonder-sk]

In some cases it may be useful to extract geoid separation (the difference between the ellisoid and orthometric height) which GNSS applied to the originally calculated ellipsoid height. This could be a new variable available to users - and it could be also reported in the GPS info page.

This information is already available in QgsGpsInformation::elevation_diff (since QGIS 3.18)

This issue has been linked to a Hellonext post: Add support for orthometric height ✨