shape() method for DataItems, RealDataItem and InstrumentItems

GUI/coregui/Models/BeamItems.cpp

GUI/coregui/Models/BeamItems.h

GUI/coregui/Models/DataItem.h

GUI/coregui/Models/DetectorItems.h

GUI/coregui/Models/InstrumentItems.cpp

GUI/coregui/Models/InstrumentItems.h

GUI/coregui/Models/IntensityDataItem.cpp

GUI/coregui/Models/IntensityDataItem.h

GUI/coregui/Models/RealDataItem.cpp

GUI/coregui/Models/RealDataItem.h

GUI/coregui/Models/RectangularDetectorItem.cpp

GUI/coregui/Models/RectangularDetectorItem.h

GUI/coregui/Models/SpecularDataItem.cpp

GUI/coregui/Models/SpecularDataItem.h

GUI/coregui/Models/SphericalDetectorItem.cpp

GUI/coregui/Models/SphericalDetectorItem.h