[RM-6023] normalize input spectrum (and noise)

Created on 2020-10-01 12:44:06 by Didier Vibert. % Done: 0

to avoid computation at the limit of IEEE floating point precision
ie 1E-18 for euclid

• needs to define a parameter with the normalization value
• decide at which level (Client or main processflow::process) convert back the ouputs (amplitudes, flux, etc...)