BREAKING
What: Introduced LegacyLabelSetImageIO readonly.
Removed old LabelSetImageIO
+ started with new MultiLabelSegmentationIO
+ added LegacyLabelSetMimeType
Why: New MultiLabelSegmentation only allows unique label values over
all layers. This is violated by all old LabelSetImages that have more then
one layer containing labels. The new legacy reader allows to read this
segmentations and either (a) addapt the pixelvalues to be compliant or
(b) keeps data untouched but loads each layer as own segmentation data.
In addition the internal format of storing the label information was changed
and simplified.
How to adapt: Use LegacyLabelSetImageIO to load old data. For everything else
use the new MultiLabelSegmentationIO.