Add component for generalised Planck law

[StePhanino]

Ciao,
I've been working with the generalized Planck law since a couple of months.

The idea is to create a series of components for Lumispy, allowing the implementation of the Generalized Planck law.

For sake of clarity, I put all the stuffs we have developed in this generalizedplanck repo. It's in the form of an installable python package where I tried to comment all functions and classes.

The idea is to discuss such a functionality, if the proposed implementation is ok, how to expand/generalize it , how to integrate it in lumispy etc.