Add robust statistics option to noise measurements
Use true ratio of exposure times to normalize plots

* measure-noise.py (flat_image):
  * Read the exposure times from the FITS header
  * Use them to calculate R0 instead of using the estimated median
    ratio from the data
Also do log-log plot of sigma vs 1/sqrt(N)

And redo all the decent pairs of exposures.
Change histogram to log scale in brightness

* measure-noise.py: And lots of other improvements too