Single-scattering model should return scattering length

[vnmanoharan]

structcol.model.reflection() should be modified to return the scattering length. At the moment it returns the transport length and asymmetry factor, which can be used to calculate the scattering length, but it makes sense to return the scattering length too. The scattering length is needed for Monte Carlo calculations, for example.

Perhaps the way to handle this is to have the function return a results object that has all the calculated quantities, including:

• parameters used to set up the calculation
• parallel, perpendicular, and unpolarized reflection fractions
• differential scattering cross-section as a function of angle
• total scattering cross-section
• asymmetry parameter
• rho (number density)
• scattering length
• transport length