[RM-7986] [linemeas] polynomial under disjoint doublet

Created on 2023-03-31 13:36:18 by Didier Vibert. % Done: 0

following comment #7216-2 of @vlebrun (3d point) , we should use 2 distincts polynomials when the 2 lines of a fixed ratio doublet (eg OIII) are well separated. Currently there is only one polynomial for each element at most (but several elements can share a unique polynomial if the elements are sharing some samples), and a fixed doublet is contained in one element since the CLineModelElement class corresponds to one amplitude to fit.