Faster phase unwrapping

[sk1p]

Right now, phase unwrapping can be quite slow, as it is a single core operation, and sometimes run on quite large images. Investigate if it's possible to speed this up, and maybe even implement this on the GPU.

[sk1p]

Assigning low priority as phase unwrapping is a more uncommon operation than many others; it would be nice for live imaging though.

[sk1p]

Assigning low priority as phase unwrapping is a more uncommon operation than many others; it would be nice for live imaging though.

... which could even use similarity between consecutive images to help performance