Update migration guide with Hough transform changes

[dzenanz]

typedef itk::HoughTransform2DCirclesImageFilter<ImageProcessingConstants::DefaultPixelType, ImageProcessingConstants::FloatPixelType> HoughTransformFilterType; now fails to compile:

47>c:\misc\bluequartz\dream3d_plugins\imageprocessing\imageprocessingfilters\itkhoughcircles.cpp(193): error C2976: 'itk::HoughTransform2DCirclesImageFilter': too few template arguments

A sentence or two about reasoning, and a suggested solution should be added to Class changes section of the migration guide.

[N-Dekker]

@dzenanz OK, I'll add a line or two on this topic to the migration guide.

By the way, this is the related commit:
06afe0f#diff-cd71664328a9fe41ea4b2c144e7ea9e5

[N-Dekker]

@dzenanz What about this one?

With ITK 5.0, HoughTransform2DCirclesImageFilter<TInputPixelType, TOutputPixelType, TRadiusPixelType> no longer has a default argument for its last template parameter. Instead, users of the filter should now explicitly specify all three template arguments. Earlier versions of ITK assumed TRadiusPixelType to be equal to TOutputPixelType, but it appears that for TRadiusPixelType, a floating point type is often preferred, whereas for TOutputPixelType, an unsigned integer type is often more appropriate.