Slice has member ZLimits; correct examples where layer had thickness zero

Sample/Processed/ProcessedSample.cpp

Sample/Scattering/ZLimits.h

Sample/Slice/Slice.cpp

Sample/Slice/Slice.h

Sample/Slice/SliceStack.cpp

Sample/Slice/SliceStack.h

Sample/StandardSamples/MagneticLayersBuilder.cpp

Tests/UnitTests/Core/Fresnel/SpecularScanTest.cpp

Tests/UnitTests/Core/Sample/RTTest.cpp