FixNullElevation: also take into account slope

For some reason, I have GPX tracks that have the first two points be
~100 meters apart in elevation, but only ~20 meters apart in distance.
This is quite unrealistic and produces pretty bad height plots (almost
as bad as the zero elevation).

Since the issue is very related, and the fix is pretty much the same, I
thought it would be a good idea to adapt the FixNullElevation
transformer to handle this case as well. For reference, "the internet"
says that the maximum slope for a MTB is ~15% to ~35%, depending on the
conditions - with 35% being pretty steep. I think it's fair to throw
away elevations that exceed 100% (basically a 45° angle upwards),
especially since we only discard them at the start and end.

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