[RM-9220] [linemeas] include correlation when computing complex uncertainties (Halpha+NII, OII, ...)

Created on 2024-09-27 12:31:12 by Didier Vibert. % Done: 0

In the current implementation the HaNII complexe or OII doublet uncertainty are computed by summing the uncertainty of each line, without taking into account the correlation between them.

Depending on the fitter used, this information is available:

• individual fitter: no join fit thus no covariance.
• svd and hybrid fitter: covariance is computed but only the diagonal is stored and used
• lbfgsb fitter: the covariance is not computed, TBD (see #6715)