add function for changing image and colormap in GUI

- make tests for wq_stack.py executable
- add WQBrowser.set_image() function
- add WQBrowser.set_cmap() function
reuse set_image in WQStackBrowser
correct update error (uncomplete merge)
Merge branch 'master' of https://github.com/rhambach/TEMareels
correct bug in fit_zlp.py introduced in last commit

- the x-axis is already rebinned and not given in px