Postfit plot with uncert band

Hi Noel,

Here are two files related to the postfit with uncert band plot.
The computation is performed in finalfit_uncertband_plot.py
and test_finalfit_uncerband_plot.py is an example on how to use it with HSG4-type workspaces.
I tried to make the computation as general as I could.

Cheers,
Quentin