implemented linear-time search for alternative cell labels
speed up of alternative label enumeration for TED
outsourced LocalToleranceFunction and DistanceToleranceFunction from TolerantEditDistance
increased verbosity of TolerantEditDistance
bugfix: _haveBackgroundLabel was set too late in TolerantEditDistance
compute 'candidateness' of cells in DistanceToleranceFunction on the fly
added headless option to edit_distance
speedup of relabel alternative search in DistanceToleranceFunction