MergeSlabs and SerialSlabs assume equilibrium wrongly

[CorentinGrimaldi]

🐛 Describe the bug

When MergeSlabs (//) is used between 2 spectra, one at equilibrium, the other one at non equilibrium, the resulting spectrum is wrongly assumed to be at equilibrium for some parameters such as radiance. See the exemple below.

When SerialSlabs (< or >) is used, the same problem occurs, and moreover the resulting spectrum is considered at equilibrium if the 2 spectra were at equilibrium (which makes no sense)

🖥 Steps To Reproduce

from radis import SpectrumFactory

sf_co2 = SpectrumFactory(wavelength_min=4160,
                         wavelength_max=4200,
                         molecule='CO2',
                         isotope='1,2,3,4',
                         mole_fraction=1e-2,
                         )
sf_co2.load_databank('CDSD-HITEMP')

sf_co  = SpectrumFactory(wavelength_min=4160,
                         wavelength_max=4200,
                         molecule='CO',
                         isotope='1,2,3',
                         mole_fraction=1e-6,
                         )
sf_co.load_databank('CO-HITEMP')

spec_co2_neq = sf_co2.non_eq_spectrum(Trot=2300, Tvib=2600)
spec_co_eq = sf_co.eq_spectrum(Tgas=2300)

spec_co2_neq.apply_slit(3, unit='nm', inplace=False).plot(wunit='nm', lw=2, Iunit='µW/cm^2/sr/nm')
(spec_co2_neq // spec_co_eq).apply_slit(3, unit='nm', inplace=False).plot(nfig='same', wunit='nm', lw=2, Iunit='µW/cm^2/sr/nm')

🎯 Expected behavior

MergeSlabs should consider the spectrum at equilibrium ONLY if the 2 spectra were at equilibrium.
SerialSlabs should NEVER consider the resulting spectrum at equilibrium.
The .is_at_equilibrium() function should be tested.

💡 Possible solutions

PR ready