GPXIN-30: Percentile Calculations

For each layer in the heirachy (i.e. segment, track and journey) a new set of stats should be generated - percentile speeds.

Ideally the stat should be an array containing a value for each percentile, so to get the 95th percentile you'd simply access position 95 in the array.

Probably worth also creating a get function so that percentiles can be grabbed directly rather than having to grab the stats object and then extract from there