BREAKING: Removed class LabelSet
Why: Introduced to much complexity for features that are not needed
anymore or could be covered with stl concepts
How to adapt: All needed functions have been moved to the LabelSetImage
class. Check LabelSetImage class interface, if use used a LabelSet method
in the past (e.g. GetLabel(...), GetLookupTable() or GetActiveLabel()).

• removed LabelSet class
• adapted LabelSetImage class