Background

This is a first implementation of the background smearing.
In addition to the dictionary of backgronds 
PyFBU.background  = {bckg1:[...],bckg2:[...]}
the user can provide a dictionary of normalization smearings
PyFBU.backgroundsyst = {bckg1: 0.10, bckg2:0.15}
where the width of the smearing to be applied is provided.

The implementation needs to be optimized...suggestions are welcome!