Layer of abstraction for postprocessing

[marcobonici]

Right now, the implemented post processing is not satisfactory for two reasons:

1. We implemented an hardcoded version of it, i.e. the value of A_s is supposed to be the first of the input arrays at the library level.
2. We are using a pretty much easy preprocessing, but we might want to include different things in the nearby future (tau rescaling or similia).

The goal of this task is to implement a different way to include the postprocessing, with a file to load (in order to make it more flexible).