Dev documentation "How to implement a new model"

[KaisaridiSofia]

As discussed with @GabrielleCasimiro, we should add a part in the documentation on how to implement a new model in Leaspy. The idea is to provide a general guide on the steps a future developper needs to follow to add his/her own model.

[MendezSebastianP]

This could be potentially useful for the new internet :) i can review it when it will be ready