Investigate noise properties of flat fields

* measure-noise.py:
  * Try to deduce the gain from the data
  * This is to make sure that I understand the noise statistics
  * So far, I am just taking the ratio of two quartz lamp frames
  * I plot the distribution of the ratio of the two images as a
    function of the mean brightness
  * On of the images has an exposure 3 times the other, which I still
    need to take account of