[RM-6489] handle the use of fft processing for QSO

Created on 2021-05-10 13:54:42 by Didier Vibert. % Done: 100

now that fftprocessing is a parameter of the method templatefitting : it can be activated for qso and stars .

But, since #5989, the zstep needs to be derived from the input spectrum sampling (when fftprocessing is activiated), it is the case for galaxies.
Thus the zstep needs to be the same for all objects. But the zgrid does not need to be aligned since the templates for each objects are different, the template sampling can be different.

And symmetrically, if the input spectrum in not log-sampled (this is not the expected behavior inside an integrated pipeline where the 2d processing should handle the log-sampling), it has to be resampled, for each different zsteps => better have a common zstep for galaxy/stars/qso

TODO: handle those aspects. note, in the current state, fftprocessing can be activated for stars and qso, and the behavior is uncertain (it may crash... or lead to incorrect results).
=> at the bare minimum: render fftprocessing impossible with stars & qso : DONE
=> or handle it correctly