Change Volume measurement to return size

This volume is the number of pixels, not the physical size so an
unsigned integer type is needed.

Change-Id: I426dcb325b76da61782f46ef945dffefbcf829cf
Adding testing of measurements for LabelGeometryImageFilter

Change-Id: Iea08923db228d0307097a208c7c2f940b44eeb24
Merge topic 'SIMPLEITK-524_AddLabelGeometryImageFilter' into next

29110d26 Adding testing of measurements for LabelGeometryImageFilter
cc4aa54f Change Volume measurement to return size