GroundTruthSegmentExtractor extracts only ContinuationSegments, now

This is more general because it allows for multi-way branches. However,
the extracted ground truth is now not consistent anymore in the sense
that segments should not overlap. This is currently only a problem for
training the RF classifier, since we don't extract BranchSegments
anymore.
bugfix: computation of section limits in SegmentTrees was wrong for continuations
SplitMerge produces Continuation and EndSegments only, now

Similar to ground truth extractor, allows for multi-way branches.
updated util
updated imageprocessing