Custom allocation and copying of distance values in dijkstra

[Luis G. Torres <lgtorres42@…>]

While the compare, combine, zero, and inf concepts allow for some nice customizable distance types in dijkstra_shortest_paths(), there is still no option to allow for custom allocation and copying of each distance value.

For instance, if I'm forced to use an object to allocate new distance values like DistAllocator::allocDist(), it's very difficult to wrap this functionality in a way that only uses default constructors and copy constructors as is currently the case in dijkstra_shortest_paths(). This typically requires creating an auxiliary class with the DistAllocator as a static class variable, which is restrictive if there are multiple DistAllocators.