Debug power spectra

[corentinravoux]

Fixing some problems in power spectrum generator module. Started by @bastiencarreres
Basis for improvement of this module

[corentinravoux]

@bastiencarreres
I change the way the normalization is done, because I do not personally like the fiducial growth rate normalization.
Just one question, why did you moved power_spectra_generator file ? I am ok with all the other changes

[bastiencarreres]

@corentinravoux The idea from my point of view was to have all pw related content in the submodule flip.power_spectra. I changed the __init__ of this submodule in order that you can do flip.power_spectra.compute_powe_spectra(). It's more an organisation thing but we can put the file at it's previous location if you prefer !

[corentinravoux]

I am ok with that but I will rename power_spectra_generator.py into generator.py for visibility, let me push some stuff