Add the @SCALFMM_PRIVATE to many files

Src/GroupTree/FGroupAttachedLeaf.hpp

Src/GroupTree/FGroupOfCells.hpp

Src/GroupTree/FGroupOfParticles.hpp

Src/GroupTree/FGroupSeqAlgorithm.hpp

Src/GroupTree/FGroupTaskAlgorithm.hpp

Src/GroupTree/FGroupTaskDepAlgorithm.hpp

Src/GroupTree/FGroupTaskStarpuAlgorithm.hpp

Src/GroupTree/FGroupTestParticleContainer.hpp

Src/GroupTree/FGroupTree.hpp

Src/GroupTree/FP2PGroupParticleContainer.hpp

Src/Kernels/Uniform/FAbstractUnifKernel.hpp

Src/Kernels/Uniform/FUnifCell.hpp

Src/Kernels/Uniform/FUnifDenseKernel.hpp

Src/Kernels/Uniform/FUnifInterpolator.hpp

Src/Kernels/Uniform/FUnifKernel.hpp

Src/Kernels/Uniform/FUnifM2LHandler.hpp

Src/Kernels/Uniform/FUnifRoots.cpp

Src/Kernels/Uniform/FUnifRoots.hpp

Src/Kernels/Uniform/FUnifSymM2LHandler.hpp

Src/Kernels/Uniform/FUnifTensor.hpp