rename RectangularDetector -> FlatDetector

Base/Pixel/RectangularPixel.h

Device/Detector/FlatDetector.cpp

Device/Detector/FlatDetector.h

Device/Detector/IDetector.h

GUI/Model/Detector/DetectorItemCatalog.cpp

GUI/Model/Detector/FlatDetectorItem.cpp

GUI/Model/Detector/FlatDetectorItem.h

GUI/Model/Device/InstrumentItems.cpp

GUI/Model/FromCore/ItemizeSimulation.cpp

GUI/Model/Model/ParameterTreeUtil.cpp

GUI/View/Device/DetectorAlignmentForm.cpp

GUI/View/Device/DetectorAlignmentForm.h

GUI/View/Device/FlatDetectorForm.cpp

GUI/View/Device/FlatDetectorForm.h

GUI/View/Instrument/DetectorEditor.cpp

GUI/View/Instrument/DetectorEditor.h

GUI/View/Instrument/OffspecDetectorEditor.h

Sim/Export/SimulationToPython.cpp

Tests/Functional/Fitting/FitTests.cpp

Tests/SimFactory/MakeSimulations.cpp