Make some segmentation fields optional

[vanossj]

I would like to use the 3DSlicer nrrd segmentation schema by adding some tags to the nrrd header.

Slicer is perfectly happy to load .seg.nrrd files that only have Color, Name, LabelValue, ID, Layer. However slicerio won't load these files if they don't contain all the metadata like Extent, ColorAutoGenerated , etc.

What is the minimum required subset of segmentation fields and does it make sense to allow slicerio to ignore missing fields if they are not required by 3DSlicer.