add speed and/or altitude data if misding

[lgasp]

This is 2 requests in one:

• speed: I tried to import a gpx that didn't contain speed data and the time-speed plot was empty. Wander could implement an option to add speed data by calculating it from points, maybe with some filtering
• altitude: my gpx files also lack altitude data. This could be calculated from world elevation data (one example here)