[RM-7138] reduce memory footprint of chisquare array in linemodel

Created on 2022-03-17 11:10:55 by Didier Vibert. % Done: 0

since #6503 the memory used by the ChisquareArray class is huge with linemodel, since the number of chi2 curves goes from lineratio_nb to lineratio_nb * continuum_nb

rmq: the memory will already be reduced with #7137

idea: store in the ChisquareArray only the continuum chi2 and linemodels (ratio) chi2, and implement an iterator that compute the chi2 for the combinations