Follow a bunch of suggestions by clazy

src/language/ast/ASTNodeDataTypeFlattener.cpp

src/language/modules/ModuleRepository.cpp

src/language/node_processor/ASTNodeListProcessor.hpp

src/language/utils/ASTNodeDataType.hpp

src/language/utils/BuiltinFunctionEmbedderUtils.cpp

src/mesh/Connectivity.cpp

src/output/GnuplotWriter.cpp

src/output/GnuplotWriter1D.cpp

src/output/VTKWriter.cpp

src/scheme/DiscreteFunctionUtils.hpp

tests/FixturesForBuiltinT.hpp

tests/test_BuiltinFunctionRegister.hpp

tests/test_CellIntegrator.cpp

tests/test_Connectivity.cpp

tests/test_DiscreteFunctionIntegrator.cpp

tests/test_DiscreteFunctionInterpoler.cpp

tests/test_DiscreteFunctionP0.cpp

tests/test_DiscreteFunctionP0Vector.cpp

tests/test_DiscreteFunctionUtils.cpp

tests/test_DiscreteFunctionVectorIntegrator.cpp