Improved launch height computation

[tobiz]

The current launch height is assumed to be the height the tug gets to provided on separation the tug then drops to a lower altitude; this is not accurate enough to use the data for costing the flights. A better algorithm would be as follows:
Since it is already determined which tug launched which glider and the tracks for both are known then by computing the 2D distance between each point in both sets of track points when the distance is greater than a specified value (set in the GUI but sensibly a function of the length of the tow rope) then the altitude of the tug at this point will be the launch height. Why only the 2D distance? Since the distance between the glider and the tug is bounded by the 3D cone determined by the length of the tow rope, it will either be the length of the tow rope or less in 2D. The 2D distance between 2 GPS coordinates can be computed by geopy.distance. Whilst it may take some time to process the 2 coordinate sets it is done after flying has ceased and hence time is not an issue. Neat! having computed the launch height very accurately this data can then be used to cost the flights (next development phase).