UnitTest/Core split in several subdirectories to allow parallel make

Tests/UnitTests/Core/0/CVectorTest.h

Tests/UnitTests/Core/0/ConstKBinAxisTest.h

Tests/UnitTests/Core/0/CustomBinAxisTest.h

Tests/UnitTests/Core/0/FixedBinAxisTest.h

Tests/UnitTests/Core/0/Histogram1DTest.h

Tests/UnitTests/Core/0/Histogram2DTest.h

Tests/UnitTests/Core/0/KVectorTest.h

Tests/UnitTests/Core/0/UtilsTest.h

Tests/UnitTests/Core/0/VariableBinAxisTest.h

Tests/UnitTests/Core/1/FormFactorTest.h

Tests/UnitTests/Core/1/LayerRoughnessTest.h

Tests/UnitTests/Core/1/LayerTest.h

Tests/UnitTests/Core/1/MultiLayerTest.h

Tests/UnitTests/Core/1/ParticleCompositionTest.h

Tests/UnitTests/Core/1/ParticleCoreShellTest.h

Tests/UnitTests/Core/1/ParticleLayoutTest.h

Tests/UnitTests/Core/1/ParticleTest.h

Tests/UnitTests/Core/2/MatrixRTCoefficientsTest.h

Tests/UnitTests/Core/2/MatrixSpecularInfoMapTest.h

Tests/UnitTests/Core/2/ScalarRTCoefficientsTest.h