CompositeSample

Hi Noel,

Could you push this into your repository ?
Then I could rebase ;-)

List of modifications:

plotting.py: added the compare and the draw_ratio functions
sample.py: added the CompositeSample class (+ some commenting in the Sample class)
In several samples subclasses, I added the log = log[__name__] ligne to have a more accurate location of the messages

shapes_syst: The driver is working but I will update it later with a better plot formating and a protection against no-systematic usage

Quentin