Current exterior label is problematic as it mixes two semantics: (1) not labeled and (2) being background (which would be labeled),

In the future (2) should be encoded by a normal label. And (1) is indicated by the pixel value 0 == LabelSetImage::UnlabeledLabelValue.

TODOs:

• Remove GetExternalLabel function and alike
• Check for all usages of "Exterior" or "0" in context of labels (as the where used in different places (e.g. IO) for checks
• Check for wrong labelset.size assumptions. The exterior label was also stored in the labelset, but never shown. Thus some codes working with LabelSetImages always ignored the first label or checked for emptiness with labelset.size()>=2 (e.g. in statistics!!!!). Double check all code for such occurences that works with LabelSetImages or LabelSets.