[RM-8012] [linemeas] restrict the least-square computation to lines support

Created on 2023-04-07 13:18:50 by Didier Vibert. % Done: 0

for current linemeas (ie not full Gaussian fit), the leastsquare term to pick the best z should be computed using only the samples under all the lines, but not the samples outside the lines since these samples are at zero in the model (the polynomials to fit the continuum are limited around each lines)