Fix unsigned int comparison warning in LabelStatistics Test

Change-Id: I11c024946a4c2a03c3d036052f6a9fde865cef7c
Remove CustomCast::Helper when no cast in needed

In the template code for the active measurements, the Helper class for
no custom_cast has been removed. The potential for an additional copy
is removed.

Change-Id: I32a740574a2324598faf4c845425b97ec52803f3
Merge topic 'SIMPLEITK-490_Uset1functionForLabelStatisticsImageFilter' into next

3a303ffc Remove CustomCast::Helper when no cast in needed
3d35c7a6 Fix unsigned int comparison warning in LabelStatistics Test