Implement the Jonker-Volgenant assignment algorithm

This implementation is based on the Fortran code provided by Jonker and
Volgenant themselves.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Add a test for the Jonker-Volgenant algorithm

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>