Interpolate tracks to find more explorer tiles

[martin-ueding]

At the moment I only look at the points in each activity to determine the explorer tiles. Other sites interpolate to make sure that tiles are also counted when no point is in them but there is a line going through it.

This requires some linear interpolation between the points to increase the resolution. We don't need to interpolate between all points and only between points where the tiles changes diagonally. That should be pretty efficient, actually.