[RM-8522] enable different redshift steps for different astronomical objects with fftprocessing

Created on 2023-12-05 13:46:55 by Didier Vibert. % Done: 0

Currently, when LineModelSolve.continuumfit.fftprocessing is set to true for several astronomical objects (eg galaxy and qso) a common z step is required since the spectrum is (re)sampled with the same step, and the faintest z step is kept.

But it is possible to enable different steps, if they have an integer ratio, using a decimated version of the spectrum. This technique is already used between firstpass and secondpass.

Due to this limitation, in the current version if fftprocessing is enabled for galaxy, it cannot be enabled for qso, because the small z step used for qso increases too much the cpu time. With this feature implemented we could gain some cpu time by enabling fft for both.