Remove explicit choice of SSCA approximation and related enum

Core/Aggregate/ILayout.cpp

Core/Aggregate/ILayout.h

Core/Aggregate/ParticleLayout.cpp

Core/Export/SampleToPython.cpp

Core/StandardSamples/SizeDistributionModelsBuilder.cpp

Examples/Demos/simul_demo_cyl_SSCA.py

Examples/Demos/simul_demo_movie3.py

...3_ExtendedExamples/experiment_at_galaxi/sample_builder.py

...imulation/ex03_InterferenceFunctions/ApproximationSSCA.py

GUI/coregui/Models/GUIDomainSampleVisitor.cpp

GUI/coregui/Models/ParticleLayoutItem.cpp

GUI/coregui/Models/ParticleLayoutItem.h

GUI/coregui/Models/TransformToDomain.cpp

auto/Wrap/libBornAgainCore.py

auto/Wrap/libBornAgainCore_wrap.cpp