Implement Stamnes' "iteration of the source-function" interpolation method as a subroutine

[dhjx1996]

While every $\mu$-interpolation of the discrete ordinates intensity function approximation will have caveats, Stamnes' "iteration of the source-function" interpolation method (see Sections 2.7 and 2.8 of Stamnes, Knut and Tsay, Si-Chee and Wiscombe, Warren and Laszlo, Istvan and Einaudi, Franco. 2000. General Purpose Fortran Program for Discrete-Ordinate-Method Radiative Transfer in Scattering and Emitting Layered Media: An Update of DISORT) is non-standard and performs uniquely well. Therefore, it should be implemented as a subroutine.

[dhjx1996]

Added a different interpolation method which is not necessarily better than the "iteration of the source-function" interpolation method" but is simple, reliable and should be good enough for now.