BREAKING!!! MaskGenerator interface was changed.
Why: Should support multiple masks. E.g. to cover mask sources that could
have overlapping labels (e.g. like the groups in MultiLabelSegmentation).
HowToAdapt:

• If one calls MaskGenerator:GetMask() the ID/munber of the mask must no be

passed. For single mask generators it is always 0.

• Change old GetMask() implementation to DoGetMask(). Also implement

GetNumberOfMasks for custom mask generators.

Additionally:
+ Added MultiLabelMaskGenerator
+ Added support of multiple LabelSetImage groups in the statistic