Docs update

I am working with gpxpy in IPython and could use some more descriptive docstings when playing around with the data. (especially nice to have when using the interactive help, e.g. `gpx.adjust_time?`). I used a pandas-style docstrings which I think look nice in the console and in IPython.

I did not update all functions, but tried to hit the high points. Please let me know if I made any mistakes (especially on distinguishing route/track/segmnent, parameter/return units, parameter/return types, and overall intention of function) and I'll try to push out fixes as soon as possible.

Thanks for the awesome library!