Wrong interpolation in TransformerSpoortakToCoordinates.py

[Neliz]

The interpolation in the transform method in TransformerSpoortakToCoordinates.py is not valid.
At the start of the geometry the lokale km is not (always) zero. So it should use the travelled distance on that geometry (difference in lokale_km from start and the point of interest).
But that would still not be a correct way to do it because than it assumes that a geocode km is linear, which is not the case. We should use M-values for this.

[JoostProRail]

Fixed in #49