add network redundancy metric

[mpadge]

Extending from discussions with @Robinlovelace & @fiftysevendegreesofrad here (private repo), in relation to the "Urban Network Analysis" software, which offers a metric of network redundancy.

Urban Network Analysis "Redundancy Index"

The importance of network redundancy has long been established in many domains [... insert lotsa citations ...], so we can't simply ignore this index. What it does it to calculate pairwise shortest paths between all points. Then for each single path from O to D, it takes a cofficient > 1 (call it k; they just call it coeff) describing a hypothetical geometric increase in permissible shortest-path distance from d to some dk > d. The script then simply loops through all edges, checks whether the distance from O to the start of the edge + the edge distance itself + the distance from the end to D is < dk. If so, that edge is redundant at the "tolerance" k. The total length of all such redundant edges for a given value of k is the network redundancy. Cool metric. Very inefficient to code - an unrestricted pairwise Dijkstra behind an O(E) loop over all edges -> O(2-2.5E) or so (roughly speaking; presuming O(V)~O(E) for street networks), but nevertheless obviously important and interesting.

An additional point of interest is likely that, particularly for automobile routing, drivers are likely to choose redundant ways, as they offer more choice should some unexpected obstacle/delay arise. The redundancy metrics are point-based, and so can be explicitly mapped on to the network and used for routing. It doesn't seem that their software offers the capability to do that, but it would be pretty straightforward to incorporate within dodgr at least - and i imagine sDNA. Not suggesting we consider it here, just recording it for posterity coz it's not something I'd not considered before, and i find it quite compelling.

[mpadge]

Closing this; can re-open later if desired