viewer: live fit of 2D Gaussian

[tgruene]

Before data collection for diffraction, we (Vienna), focus the direct beam on the detector. The intensity can be seen on about 4-8 pixels. This makes it difficult to get the focus perfect (although it is usually very good). It would be great to have a live view next to the viewer canvas, that show the amplitude, sigma_x and sigma_y for a 2D gaussian fitted live to the direct beam (or region of interest). It woudl speed up data collection by a couple of important seconds.