IDetector now returns vector of allowed detector units

Core/Algorithms/inc/IDetector2D.h

Core/Algorithms/inc/RectangularDetector.h

Core/Algorithms/inc/SphericalDetector.h

Core/Algorithms/src/IDetector2D.cpp

Core/Algorithms/src/Instrument.cpp

Core/Algorithms/src/RectangularDetector.cpp

Core/Algorithms/src/SphericalDetector.cpp

GUI/coregui/Models/IntensityDataItem.cpp

GUI/coregui/Models/JobItem.cpp