Add speed to gpx if missing

[FlorianWilhelm]

First of all gpxpy is really great and I like that I can add the elevation so easily to a track with the help of srtm.py. I would be cool to have this feature also for the speed. There is get_moving_data that actually calculates what I want but does not return it. A function add_missing_speed would be nice that calculates and adds the smoothed speed to all the GPXTrackPoint points. A simple way of smoothing would be to assign the calculated speed from time and distance to the center of the line between to GPXTrackPoints. The speed at a GPXTrackPoint is now the weighted speed of the two adjoining lines. For the first and last GPXTrackPoint one would need to assume 0 or a cyclic boundary condition.

[tkrajina]

Yes, I agree. I don't have the time to implement this properly now, but if you want to experiment, I just pushed a quick idea. It's in 7032e0f (branch: add-missing-speed). In the _add(...) function interval should contain points without .speed, start and end are points with .speed before and after the interval.

This (if it works) should only add missing speeds and it is not especially smart.

The smoothing should maybe be done in smooth() by changing:

    def smooth(self, vertical=True, horizontal=False, remove_extremes=False):

to:

    def smooth(self, vertical=True, horizontal=False, remove_extremes=False, speed=False):

And then implementing it in there.