[FEAT]: Add wrapper functions for soft x-ray index of refraction.

[HarlanHeilman]

It would be nice if the popular index of refraction components $\delta$ and $\beta$ could be added as functions to the xsf module. They could be modeled from the index of refraction function.

Proposed addition:

def delta(compound, *, density=None, natural_density=None,
                        energy=None, wavelength=None):
    """
    Calculates the δ component of the index of refraction for a given compound
    ...
    """
    if energy is not None:
        wavelength = xray_wavelength(energy)
    assert wavelength is not None, "scattering calculation needs energy or wavelength"
    f1, f2 = xray_sld(compound,
                      density=density, natural_density=natural_density,
                      wavelength=wavelength)
    return wavelength**2/(2*pi)*(f1)*1e-6

def beta(compound, *, density=None, natural_density=None,
                        energy=None, wavelength=None):
    """
    Calculates the β component of the index of refraction for a given compound
    ...
    """
    if energy is not None:
        wavelength = xray_wavelength(energy)
    assert wavelength is not None, "scattering calculation needs energy or wavelength"
    f1, f2 = xray_sld(compound,
                      density=density, natural_density=natural_density,
                      wavelength=wavelength)
    return -wavelength**2/(2*pi)*(f2)*1e-6

Should I make a pr for this?