Add converters from RAI to Img and II to Img

[ctrueden]

Converters would be a good basis for convert ops that go from RAI to Img and II to Img. @awalter17 and I think we would need six initially: RAIToArrayImg (only matches if element count is less than maxint), RAIToCellImg (but with what cell size?), RAIToListImg, and then those three again for II, and in that order of priority. This would let us implement convert.imgView.

But what about create.img? It would be nice to reuse the logic above, but only for factories. We could make e.g. RAIToImgFactory converters like RAIToArrayImgFactory, which would obviously be lossy. But is this really a "conversion"? Would someone want to pass a RAI as an argument to something that takes an ImgFactory? I don't think so.

Migrated from #72 (comment)

[imagejan]

For the record: there is now ImgView.wrap(rai) which chooses a suitable ImgFactory, so a converter could simply make use of this, right?

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and II to Img

The problem with IterableInterval is that it "is not guaranteed to iterate over all coordinates of its containing Interval.". Do we actually want to have a converter for this?

[ctrueden]

I'm going to close this in favor of #103. In that PR, I added converters for all image types to/from one another. That does not include IterableInterval, though—I agree with @imagejan that we cannot (or at least should not) write a converter from IterableInterval generally to any of the RAI-based image types. I'm also going to ignore my musings above about ImgFactory for the time being.