Implement 2D (eliptical) gaussian in magnet_logic and display the contour lines

[drogenlied]

Migration: trac#156.

The picture of the magnet alignment is often a asymmetric/elliptical looking like Gaussian distribution (not just a simple 2D gauss with the same extent in all directions).
Most probably the fit already exists (look in the 2D fit which is used in the optimizer).
The matrix is saved in the instance variable _2D_data_matrix.
The fit procedure has to be implemented in the logic as well as the GUI needs the ability to apply that fit and display 2D fit with contour lines together with the scanned image.