[RM-9443] improve bad continuum switch for high SNR spectra

Created on 2025-01-23 08:22:34 by Didier Vibert. % Done: 0

cf #9222-2

with high SNR spectra the continumm fit can have high reduced chi2 because of a low noise variance and the template intrisic noise since the templates are not perfect.

To mitigate this issue we can try to add a template intrisic noise term in the least-square denominator. This noise term could be evaluated by estimating the variance of the fitted portion of the template, outside lines.