[RM-8550] [warning] add a warning when the fftprocessing is activated twice

Created on 2023-12-18 14:18:59 by Didier Vibert. % Done: 0

when the fftprocessing is activated (with TemplateFttingSolve or LineModelSolve) twice, ie for two categories (galaxy/star/qso/...) and when the input spectrum is not logarithmically sampled in wavelength, the spectra will be resampled according to the smallest redshift step, thus the redshift grid will be common for the two categories, potentially increasing substantially the cpu time.

=> add a warning when this happens.

conditions triggering the warning:

1. fftprocessing activated for at least two categories
2. input spectrum not log-sampled
3. the redshift grid step of the two categories are not the same