correct prefactor for scattering from rough interfaces (error in Schlomka et. al, fixes #553)

Joachim Wuttkerequested to merge
i553a into r20.1

Correct prefactor k^4 instead of k^2 for scattering from rough interfaces. Error came from our literature source, Schlomka et al, 1995, Eq 3.

This resolves #553 (closed).

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