Achievements: Add PRs for Max speed

[P_C]

I would love to see some holistic speed metrics captured, namely a 2 new PRs for max speed and a graph that tracks your average speed against all rides. 

For the max speed PRs I’d like to see one for max speed on a decent where the app quantifies this by seeing the terrain at a certain negative degree %

For the second PR I’d like to see it captured as a sprint where the app records based on duration and a gradient that is far or only slightly declined / inclined. 

both of these PRs would allow you to see how you’ve either grown in confidence on descents or improved your sprint performance. 

The average speed chart should should show over a period your average speed taken from each ride as opposed to just matched rides. Whilst the terrain and conditions can vary which would impact average speed I feel it would be useful to see a trend over time.

[Jane]

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[anchskier]

The big problem I see with anything related to a max speed is that you would have to find a way to overcome anomalies in the data.  It isn't uncommon to have a ride where it shows a speed way above anything I could ever achieve due to a bad signal/data point.  You would be stuck with some anomaly as your PR for top speed rather than something realistic.  I've had MTB rides where it said I went over 100mph as my max speed, yet I clearly never came close to that anywhere on the ride.

[Jan_Mantau]

Maybe the data anomaly problem can be smoothed by using not the peak gps speed but some highest average over 10 seconds or so. But I don't see max speed on decents as an achievement anyway. All max speeds I know of are reached without pedaling so what's the point of it?