BUG: Fix itkFrequencyImageRegionIterator

[phcerdan]

Test it via recently added gtest (see #4827)

Closes #4828

PR Checklist

• No API changes were made (or the changes have been approved)
• No major design changes were made (or the changes have been approved)
• Added test (or behavior not changed)
• Updated API documentation (or API not changed)

[dzenanz]

LGTM

[N-Dekker]

Thanks @phcerdan but are those two iterators really useful? Given the fact that these two did not compile in the six years of their existence, why not remove them by now?

If they are really useful (next to all the other iterator types that we have already in ITK), can you please add a test or example to show their usefulness?

[phcerdan]

Hi @N-Dekker, ITK is a library, not an application, we provide filters that are useful for others. This iterator in particular is a really simple extension of the image iterator to provide the GetFrequency() functions that are used for FrenquencyFunctors.

This iterator in particular captures the rare (but existing, I have worked with those images from diffusion experiments in optics) where the input image is already in the frequency domain.
The more common case is to have an image in the spatial domain and perform an FFT, that is captured by the other frequency iterators.

It is here for completion on the frequency iterators, and ability to work with this kind of images.

The frequency iterators have all the same interface, and it is tested, I don't think it is necessary to add an extra test. beyond the instantiation test that is already in place, for this one, which is the simplest.

[N-Dekker]

@phcerdan OK, thanks for your explanation. Then I just hope that these two iterators are really going to be used! In general I believe it's a good thing to regularly clean-up the library, and remove unused components.

Can you please still run clang-format, and amend (+ force-push) your commit?

[phcerdan]

Done