[RM-8536] [linemodel] force absorption lines to be null when continum under line is null or negative

Created on 2023-12-11 15:29:24 by Didier Vibert. % Done: 100

in CLineModelFitting::GetModelSolution, to compute the absorbed flux one need to know the level of the continuum (since the fitted absorption amplitude in [0,1] is a fraction of this level). Currently this continuum level is measured on one pixel (the nearest from the line center). If this level is found negative, the absoprtion should be forced to null.

note: this appears essentially when setting the @continuumreestimation@ option (to "always" or "onlyextrema" )