Fix the SegmentMergingCostMatrixCreator

For it to work, iteration order must be really well controlled.
Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>
WIP: general Jaqaman cost matrix creator for track segments.

Signed-off-by: Jean-Yves Tinevez <jean-yves.tinevez@pasteur.fr>