implementing a bandpass model module

[giuspugl]

Starting from what has been implemented by @tomma90 , i essentially merged his module into mine . Few functionalities are still to be ported , but the bulk of the work is there.
the band models are defined as class and the user can decide which model to use :

• top -hat (with 2 apodization profiles, cosine and exp )
• chebyshev prototypes

Ones the class is initialized, we can do extra actions e.g. build the sampler to generate random realizations of the band (already in place) and include high freq. for out_of band analysis .

below a comparison of two bandpasses buillt from a bandmodel from Chebyshev bandpass and a Top-hat with exp apodization.