Should croproi strip rois?

[jimfinnis]

At the moment it doesn't - it crops images down to their ROI but leaves the ROI there. But when you're just looking at an image subset it can be useful to strip rois. You can do this with an extra node, though.

[jimfinnis]

So now, as the commit says, rather than throwing away all the old ROIs and making a new one which is the union, we make copies of the ROIs and move them. This works rather better.