Employ using declarations to improve code style

GUI/coregui/Models/BeamItem.cpp

GUI/coregui/Models/DetectorItems.cpp

GUI/coregui/Models/FitParameterProxyModel.cpp

GUI/coregui/Models/GUIObjectBuilder.cpp

GUI/coregui/Models/MaterialItem.cpp

GUI/coregui/Models/MesoCrystalItem.cpp

GUI/coregui/Models/ParticleItem.cpp

GUI/coregui/Models/SessionModel.cpp

GUI/coregui/Models/TransformFromDomain.cpp

GUI/coregui/Models/TransformToDomain.cpp

...regui/Views/ImportDataWidgets/RealDataSelectorActions.cpp

Tests/UnitTests/GUI/TestMapperCases.h

Tests/UnitTests/GUI/TestMapperForItem.h

Tests/UnitTests/GUI/TestParticleCoreShell.h

Tests/UnitTests/GUI/TestParticleItem.h