Use the Threshold-variable everywhere
Rewriting how images are read/selected/displayed

- Reading images in the loop and calculating mean/max/std on the fly
  with list comprehension
- Displaying all the data if and when necessary
- Shuffling around when display of image and calculation happens to
  speed up script in the case of no-image-detection
Now with 100% less crashyness!

- Using named figures, plt.ion() and clearing variables at the end helps
  with the MemoryFaults, i.e. we now can look at all the folders at
  once.
- If *no* image is above the threshold, use the brightest one as
  workaround. We need to somehow note that in the log-file and/or in the
  filename of the results.
Numbering figures and closing them --> no crash

- Handling the figures with numbers and closing them one after the other
  takes care of the crashing, i.e. we now can run the script on (the
  currently) *all* 150 experiment folders.
- Writing info about the brightest image (if above threshold) a bit more
  to the front of the log file, at the cost of another loop
- Use the correct counters everywhere
Consistent numbering in log & output
Let it rip!
Merge branch 'dark-flat-calculation'