Remove include guards. This resolves #1735.

Tests/UnitTests/Core/Axes/CVectorTest.h

Tests/UnitTests/Core/Axes/ConstKBinAxisTest.h

Tests/UnitTests/Core/Axes/CustomBinAxisTest.h

Tests/UnitTests/Core/Axes/FixedBinAxisTest.h

Tests/UnitTests/Core/Axes/Histogram1DTest.h

Tests/UnitTests/Core/Axes/Histogram2DTest.h

Tests/UnitTests/Core/Axes/KVectorTest.h

Tests/UnitTests/Core/Axes/LabelMapTest.h

Tests/UnitTests/Core/Axes/VariableBinAxisTest.h

...UnitTests/Core/DataStructure/IntensityDataFunctionsTest.h

Tests/UnitTests/Core/DataStructure/LLDataTest.h

Tests/UnitTests/Core/DataStructure/OutputDataIteratorTest.h

Tests/UnitTests/Core/DataStructure/OutputDataTest.h

Tests/UnitTests/Core/Detector/DetectorMaskTest.h

Tests/UnitTests/Core/Detector/PolygonTest.h

Tests/UnitTests/Core/Detector/PrecomputedTest.h

Tests/UnitTests/Core/Detector/RectangularDetectorTest.h

Tests/UnitTests/Core/Detector/SpecialFunctionsTest.h

Tests/UnitTests/Core/Detector/SphericalDetectorTest.h

Tests/UnitTests/Core/Fresnel/MatrixRTCoefficientsTest.h