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)
add helper function for reading many MSA files
fix bug in angular correction !

- the origin of the energy axis for the angular correction was
  always the first pixel of the x-axis, not the ZLP position
.
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
Merge pull request #1 from rhambach/master

pull changes from rhambach/TEMareels