Support newer MRML Segmentation Nodes

[blowekamp]

Add support for inputs to come from Segmentation MRML nodes. These can be converted to LabelMaps on the fly.

@pieper @lassoan Do you have links to examples/modules of how to do this?

[pieper]

See how it's done in SlicerRadiomics:

https://github.com/Radiomics/SlicerRadiomics/blob/master/SlicerRadiomics/SlicerRadiomics.py#L132