Investigate whether itkTestingComparisonImageFilter is needed

[fedorov]

This class is already present in ITK: https://github.com/Slicer/ITK/blob/c390a4e7d571802c6e7f554a9e928c3d2de4fd07/Modules/Core/TestKernel/include/itkTestingComparisonImageFilter.h.

Related to #53

[fedorov]

Ah, I see - that module is not present in the stripped-down fork of ITK that we used for dcmqi due to appveyor time limit: https://github.com/QIICR/ITK-dcmqi/tree/stripped-down/Modules/Core, which is also used in the PkModeling superbuild!