GUI: added bounding boxes to DetectorScene

Resolves #144 (closed), #115 (closed), #40 (closed)

I have added the bounding box to peaks in the detector scene, which I believe to be equiavlent to the integration region (correct me if I'm wrong).

Control of whether the peak centers or bounding boxes are displayed is handled by the new PeakViewWidget, which I have added to all subframes with a DetectorScene, and appears in the left hand panel (_left_layout). This widget allows selection of the peak and bounding box sizes and colours, and whether they are displayed or not. Not that the bounding box size spin box does nothing yet because I haven't decided what it should do (apparently the peak size is just a plotting convenience and is nothing to do with the actual peak size, in contrast with the bounding boxes).