Region of interest in Core

Core: ThreadedComputation -> InnerCounter. Manual: revised wave progation in 2+1D.

use multiple inheritance with scalar/polarized base strategies to get rid of quite some code duplication in the interference function machinery

real for the case of a vacuum top layer) is not a choice but results
from continuity conditions.
Also lots of additions to the index.

manual: improvements here and there, and revised chapter on Born approximation

local variable (thanks Walter).

longer contained in the refractive index.

interference functions: little simplifications, and improved comments

Started revision of scattering theory.