[webdoc120] reference and API simplification for MultiLayer, Layer, ParticleLayout ()

Sample/Aggregate/ParticleLayout.cpp

Sample/Aggregate/ParticleLayout.h

Sample/Multilayer/Layer.h

Sample/Multilayer/MultiLayer.cpp

Sample/Multilayer/MultiLayer.h

Sample/Scattering/ISampleNode.h

Sample/StandardSamples/LayersWithAbsorptionBuilder.cpp

Sample/StandardSamples/ParacrystalBuilder.cpp

Sample/StandardSamples/ParticleInVacuumBuilder.cpp

Tests/Unit/Sim/ParticleLayoutTest.cpp

auto/Wrap/libBornAgainSample.py

auto/Wrap/libBornAgainSample_wrap.cpp

auto/Wrap/libBornAgainSample_wrap.h

hugo/content/ex/_index.md

hugo/content/ex/gisas-no-dwba-terms/CylindersInBA_setup.jpg

hugo/content/ex/gisas-no-dwba-terms/index.md

hugo/content/ex/gisas/CylindersInDWBA_setup.jpg

hugo/content/ex/gisas/_index.md

hugo/content/ex/sas/_index.md

hugo/content/ex/sas/polarized/index.md