[RM-8638] [linemodel] wrong total EmissionLinesSNR computation (and total StrongEmissionLinesSNR)

Created on 2024-02-05 14:38:01 by Didier Vibert. % Done: 0

The total SNR of emission lines (output as "EmissionLinesSNR") is wrongly computed, since the integrated flux of all emission lines is computed by subtracting the continuum from the observed spectrum, whereas the absorption lines fitted model should also be subtracted.
cf @CLineModelFitting::getCumulSNRStrongEL()@ and @CLineModelFitting::getSNROnRange(Int32Range idxRange)@

Moreover, the summation should follow a numerical integration like trapezoidal (using for instance @CSpectrumModel::integrateFluxes_usingTrapez@).
The simplest fix would be to use the method @CLineModelFitting::getFluxDirectIntegration@ with the corresponding list of elements

(bug detected when looking at the IT change of #6459)