Use Jonker-Volgenant instead of Munkres-Kuhn

Already with the dense Jonker-Volgenant, there is a substantial speed-up relative to Munkres-Kuhn (while the result is identical, as tested by the new unit test).

In this humble (*cough, cough*) developer's tests, running 10,000 random 100x100 assignments took 8.2 seconds with Munkres-Kuhn and 2.6 seconds with Jonker-Volgenant.

So maybe we should just skip Munkres-Kuhn (which has a bug anyway because it fails to return a sensible assignment for 1x1 matrices)?