Support for single frame segmentation

[emelalkim]

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During our work with segmentation creation with @muakdogan, NormalizeMultiframe rejects single frame segmentations with "Single frame instance of multiframe class not supported" error message. We have seen use cases where people just want to segment single slice, whether to use as a seed for auto segmentation algorithms or as they are only interested in the largest section of the lesion. What is the reasoning behind this limitation? Any ideas @pieper? Can we make a PR to remove it?

[pieper]

Looks like @swederik added this, but I'm not sure what the reasoning was.

https://github.com/dcmjs-org/dcmjs/blame/42a0a3d03b6eb14335e00392e0844c4bd41c50c5/src/normalizers.js#L294

Yes, we should certainly handle the case of a single slice segmentation because it does come up pretty often.

@emelalkim I suppose the best approach is to remove this if block and see what fails, and then we can decide the right behavior. Probably it has to do with determining the slice spacing / thickness which is a weak point of SEG, but something we need to address.