Add speed to gpx if missing about gpxpy

[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.