Bug fix in WaehlbySplitter: Transitive dependencies not resolved

When setting a high distance merge threshold, already merged objects
may be merged again, but with a different label. Therefore their actual
label is not present anymore in the labeling but still in the list of
merged labels. This leads to a NullPointerException.
WaehlBySplitterOp: Minor code cleanup
Merge pull request #80 from knime-ip/waehlby_fixes

Fixes transitive dependency bug of already merged labels