Self-validation using sub-trace O/D pairs

[kpwebb]

As discussed with @abyrd we can self-validate the traffic model (+routing) by comparing observed O/D travel times against estimated travel times using the traffic model.

By sampling O/Ds from within traces we can avoid privacy issues since the O/D paris don't correspond to actual movements.

Concerns were raised by @mattwigway about sample bias as sub-trace O/Ds may include stopovers or non-direct routes since the sub-trace sections do not correspond with purposeful movements

I think there's a possibility for overcoming this by storing O/D, time and distance. Sub traces where distances aren't in-line with typical shortest path distances could be discarded, eliminating many non-direct routes. Also trips where times don't correspond with other similar observations could be discounted.