Add a plot of a histogram of the QSpace values in QSpaceView
This MR adds a tab with an histogram of the QSpace values to the QSpaceView.
The histogram on data > 0 and with bins equally spaced in log10 scale (and displayed with a log scale on the X axis).
As it is now, the histogram is updated each time the displayed data is changed. If there are issues with performance, this could only be done when the histogram widget is visible.
closes #50 (closed)