Change SessionItem::item<> return type to T* for uniformity with other methods

GUI/coregui/Models/BeamItems.cpp

GUI/coregui/Models/DepthProbeInstrumentItem.cpp

GUI/coregui/Models/DetectorItems.cpp

GUI/coregui/Models/GUIDomainSampleVisitor.cpp

GUI/coregui/Models/InstrumentItems.cpp

GUI/coregui/Models/JobItem.cpp

GUI/coregui/Models/MaterialItem.cpp

GUI/coregui/Models/MesoCrystalItem.cpp

GUI/coregui/Models/ParticleItem.cpp

GUI/coregui/Models/RealDataItem.cpp

GUI/coregui/Models/RectangularDetectorItem.cpp

GUI/coregui/Models/SessionItem.cpp

GUI/coregui/Models/SessionItem.h

GUI/coregui/Models/SpecularBeamInclinationItem.cpp

GUI/coregui/Models/TransformFromDomain.cpp

GUI/coregui/Models/TransformToDomain.cpp

GUI/coregui/Views/InstrumentWidgets/DetectorMaskDelegate.cpp

GUI/coregui/Views/SampleDesigner/ParticleView.cpp

GUI/coregui/Views/TestView.cpp

Tests/UnitTests/GUI/TestLinkInstrument.cpp