Generalize edge weight types usable in prim_minimum_spanning_tree

[lgtorres42@…]

The current implementation of prim_minimum_spanning_tree makes the following assumptions about edge weight types:

+ weights have the less-than (<) operator defined

+ weight identity value is 0

+ the maximum weight value is accessible by std::numeric_limits<W>::max()

I've attached a patch that removes these requirements of edge weights by allowing users to use the distance_compare, distance_zero, and distance_inf named parameters. These parameters are passed to the dijsktra_shortest_paths() call that underlies the prim_minimum_spanning_tree function.

[lgtorres42@…]

general edge weight patch to BGL prim's minimum spanning tree

[lgtorres42@…]

Note: adding this patch would require updating the Prim's documentation to reflect the new functionality.