peak graphics items persist even when out of frame

In DetectorScene, a PeakGraphicsItem is created for each peak in the frame. However, the items are not deleted when they go out of frame. The result, is that if one scrolls through the frames, memory usage gradually increases. For something like biodiff, which typically has a single dataset with ~100000 peaks, this can become a serious problem, with memory usage growing to 15GB or more (!).