SciView: fix removing volumes

[frauzufall]

I cannot remove volumes with the latest version on master, this fixes it. It will not remove more than entry from the imageToVolumeMap though, not sure if it should be able to do that.

[kephale]

It does need to remove multiple entries from imageToVolumeMap. I should clarify that somewhere.

imageToVolumeMap can have multiple keys point to the same volumes (e.g. a SourceAndConerter and an IterableInterval for the same image can point to the same volume)

[kephale]

@frauzufall yeah, makes sense. Silly original implementation :X

[frauzufall]

Thanks - I changed it to be able to remove multiple keys - it still fixes the issue I had with it iterating over a map while removing entries from the map.