Feature request: pace option for speed

[davidedelvento]

I've implemented a crude way to display pace instead of speed in the speed-distance and speed-time graphs of the track properties.

For reasons too long to be explained here, runners use pace instead of speed as a way to determine how fast they are going -- this is a second example of #26

I'll be happy to make my crude implementation better and submit a PR, if you'd be interested in it (just for myself, the crude version is fine). I need some simple guidance on how would be best to proceed: strictly speaking pace and speed are two different things, however many people use them interchangeable such in my speed in that race was 7' per mile. Given this state of affair, I could implement pace as two different units for the speed (minutes per mile and minutes per km) and add it globally there like the km/h, m/s and mi/h.

[davidedelvento]

Fixed by PR #33. Let me close this and keep any discussion (if necessary) there