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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def solar_spectrum(model='SOLAR-ISS'):
    r'''Returns the solar spectrum of the sun according to the specified model.
    Only the 'SOLAR-ISS' model is supported.

    Parameters
    ----------
    model : str, optional
        The model to use; 'SOLAR-ISS' is the only model available, [-]

    Returns
    -------
    wavelengths : ndarray
        The wavelengths of the solar spectra, [m]
    SSI : ndarray
        The solar spectral irradiance of the sun, [W/(m^2*m)]
    uncertainties : ndarray
        The estimated absolute uncertainty of the measured spectral irradiance  
        of the sun, [W/(m^2*m)]

    Notes
    -----
    The power of the sun changes as the earth gets closer or further away.
    
    In [1]_, the UV and VIS data come from observations in 2008; the IR comes
    from measurements made from 2010-2016. There is a further 28 W/m^2 for the
    3 micrometer to 160 micrometer range, not included in this model. All data
    was corrected to a standard distance of one astronomical unit from the Sun,
    as is the resultant spectrum. 
    
    The variation of the spectrum as a function of distance from the sun should
    alter only the absolute magnitudes.
    
    [2]_ contains another dataset.
    
    Examples
    --------
    >>> wavelengths, SSI, uncertainties = solar_spectrum()
    
    Calculate the minimum and maximum values of the wavelengths (0.5 nm/3000nm)
    and SSI:
        
    >>> min(wavelengths), max(wavelengths), min(SSI), max(SSI)
    (5e-10, 2.9999000000000003e-06, 1330.0, 2256817820.0)
    
    Integration - calculate the solar constant, in untis of W/m^2 hitting
    earth's atmosphere.
    
    >>> np.trapz(SSI, wavelengths)
    1344.802978238

    References
    ----------
    .. [1] Meftah, M., L. Damé, D. Bolsée, A. Hauchecorne, N. Pereira, D. 
       Sluse, G. Cessateur, et al. "SOLAR-ISS: A New Reference Spectrum Based 
       on SOLAR/SOLSPEC Observations." Astronomy & Astrophysics 611 (March 1, 
       2018): A1. https://doi.org/10.1051/0004-6361/201731316.
    .. [2] Woods Thomas N., Chamberlin Phillip C., Harder Jerald W., Hock 
       Rachel A., Snow Martin, Eparvier Francis G., Fontenla Juan, McClintock
       William E., and Richard Erik C. "Solar Irradiance Reference Spectra 
       (SIRS) for the 2008 Whole Heliosphere Interval (WHI)." Geophysical
       Research Letters 36, no. 1 (January 1, 2009).
       https://doi.org/10.1029/2008GL036373.
    '''
    if model == 'SOLAR-ISS':
        pth = os.path.join(folder, 'solar_iss_2018_spectrum.dat')
        data = np.loadtxt(pth)
        wavelengths, SSI, uncertainties = data[:, 0], data[:, 1], data[:, 2]
        
        wavelengths = wavelengths*1E-9
        SSI = SSI*1E9
        
        # Convert -1 uncertainties to nans
        uncertainties[uncertainties == -1] = np.nan
        
        uncertainties = uncertainties*1E9
    return wavelengths, SSI, uncertainties