Rework and optimize computation of roughness crosscorrelation

1. Now intermediate interfaces explicitly participate in computation of crosscorrelaton terms:

---------
--------- i
---------
---------
---------
--------- j
---------
---------
--------- k
---------

spectrum_i_k = spectrum_i_j * spectrum_j_k / spectrum_j

2. Computation of cross-interface scattering on roughness is optimized.

Now it is several times faster due to caching.

added 1 commit

• c6997442 - do full compulation in crossCorrSpectrum

Compare with previous version

added 1 commit

• 50f94e08 - add assert

Compare with previous version

changed title from Optimize computation of cross-interface scattering on roughness to Rework and optimize computation of roughness crosscorrelation

changed the description

added 1 commit

• 8a954009 - rename

Compare with previous version

mentioned in commit bbe50f27

merged