Support multiple targets for Suurballe

[Alpar Juttner]

The concept is that

run(s,t,k);

would do just as it does now, but in addition to it, there would be a function

init(s);

performing a full Dijkstra and computing reduced arc costs, which could be then followed by several

start(t,k);

execution, each of them would reverse the arcs on the shortest s-t path found by init(s) and execute partial Dijkstra to t only k-1 times.

It could speed up the use cases when k arc-disjoint s-t paths are needed for a lot of t nodes (e.g. all t!=s nodes).