Implement model of growing roughness (#969)
 Adds linear growth model to BA.

Roughness spectrum and rms are not the local properties of the interface anymore. The whole underlying structure is needed to compute interface properties. That is why methods
layerInterfaceSpectrum(...)
andlayerInterfaceRMS()
now belong to classMultilayer
. 
SpatialFrequencyCrosscorrelation
model now uses expression
spectrum_i_k = sqrt(spectrum_i * spectrum_k)
for crosscorrelated spectrum, instead of the old
spectrum_i_k = 0.5* (rms_k/rms_i * spectrum_i + rms_i/rms_k * spectrum_k)
The latter expression is not selfconsistent and may lead to small negative scattered intensity under certain conditions.
The new model obeys the following general relation which is also a criterion for the proper model:

 i


 j


 k

spectrum_i_k = spectrum_i_j * spectrum_j_k / spectrum_j
 Roughness rms is retrieved now by methods with
rms
in their name.Sigma
is unwanted for this role, because for the cut spectrum the sigma parameter of Kcorrelation model does not correspond the full rms.
Resolves #969 (closed)
Edited by Mikhail Svechnikov