Add smoothing functionality

[SlowMo24]

We use the start and end point of our paths to estimate the slope. For very short paths, we will get a very high slope in case the paths crosses a pixel-boundary (see red path in image)

while other short paths get a slope of 0 in a very steep terrain, because they don't cross a pixel border (see blue stairs at Tingstätte):

.

It would be nice to have a smoothing functionality on the server so the values are adjusted to e.g. 9 nearest neighbours IDW or similar.