Differentialregularization

Re-implementation of the regularization to allow the application of Tikhonov to subranges of the distribution (useful for multidimensional unfolding).

A regularization object needs to be created and fed to the fbu object. This will allow also more flexibility for implementing custom regularization functins on the user end.

    from fbu import Regularization
    pyfbu.regularization = Regularization('Tikhonov',ntotbins=6,ndiffbins=2)
 
I still need to implement a test for this