Seeded watershed performance

[PerretB]

Improve efficiency of labelsitation_seeded_watersed

Higra/include/higra/algo/watershed.hpp

Line 124 in f85538d

auto labelisation_seeded_watershed(

Idea 1: sure, shortcut first loop when num_vertices - num_seeds edges have been processed (edge case, num_seeds = 0)
Idea 2: to test, replace relabelling of nonlabeled vertices with union-find by a connected component labelling