Use string constant for PointwiseAxisItem type name

GUI/Models/BeamItems.cpp

GUI/Models/GroupInfoCatalog.cpp

GUI/Models/GroupItem.cpp

GUI/Models/GroupItem.h

GUI/Models/InstrumentModel.cpp

GUI/Models/ItemCatalog.cpp

GUI/Models/JobModel.cpp

GUI/Models/JobModelFunctions.cpp

GUI/Models/PointwiseAxisItem.cpp

GUI/Models/PointwiseAxisItem.h

GUI/Models/SessionItem.h

GUI/Models/SpecularBeamInclinationItem.cpp

GUI/Views/InstrumentWidgets/InstrumentViewActions.cpp

Tests/UnitTests/GUI/TestSavingSpecularData.cpp