Add `get_moving_speed_data` Method

Hi, again :wink: 

I (additionally) needed the ability to create a speed profile of gpx files ... Therefore I've extended your code and implemented a `get_moving_speed_data` method.

Are you - in general - interested in this pull request? Because I am not sure if it fits your coding style perfectly and I also think that there are - now - two methods missing (in terms of the completeness): `get_moving_speed_imp` and `get_moving_speed_data_imp`
... and I am not sure if I can find some time soon to add them :confused: 

What do you think?