[enhancement] Correction of the H2O cross-section for use with MT-CKD continuum

[tannoudji]

The MT-CKD continuum has been added to HITRAN, but HAPI does not yet have a direct way of calculating the H2O cross-section without the baseline at 25cm-1, which is necessary to then add the continuum.

I have implemented this in the code with 3 sadditional lines, and would suggest that add it to HAPI official code. Simply add an option in the absorption Coefficient_Generic(..., do_remove_baseline=False)

and within the code, you can replace:

            # calculate profile on a grid            
            BoundIndexLower = bisect(Omegas,TRANS['nu']-OmegaWingF)
            BoundIndexUpper = bisect(Omegas,TRANS['nu']+OmegaWingF)
            PARAMETERS['WnGrid'] = Omegas[BoundIndexLower:BoundIndexUpper]
            lineshape_vals = profile(**PARAMETERS)
            Xsect[BoundIndexLower:BoundIndexUpper] += factor * lineshape_vals

by:

            # calculate profile on a grid            
            BoundIndexLower = bisect(Omegas,TRANS['nu']-OmegaWingF)
            BoundIndexUpper = bisect(Omegas,TRANS['nu']+OmegaWingF)
            PARAMETERS['WnGrid'] = Omegas[BoundIndexLower:BoundIndexUpper]
            lineshape_vals = profile(**PARAMETERS)

            if do_remove_baseline and len(lineshape_vals) > 0:
                baseline = min(lineshape_vals[0], lineshape_vals[-1])
                lineshape_vals = lineshape_vals - baseline
            Xsect[BoundIndexLower:BoundIndexUpper] += factor * lineshape_vals

I do not test that the OmegaWingF is 25cm-1 to keep this function as general as possible. The user should be aware of this.