Merge branch 'experimental' into feature/3.0/index_tree

QUICKSTART.md

checks/check_2d.cpp

checks/check_l2p.cpp

checks/check_morton_index.cpp

checks/check_source-target.cpp

checks/debug_omp.cpp

checks/test_block.cpp

checks/test_build_tree.cpp

checks/test_chebyshev.cpp

checks/test_m2l_sym.cpp

examples/tutorial.cpp

include/scalfmm/container/block.hpp

include/scalfmm/interpolation/interpolator.hpp

include/scalfmm/matrix_kernels/gaussian.hpp

include/scalfmm/operators/fmm_operators.hpp

include/scalfmm/operators/p2p.hpp

include/scalfmm/parallel/mpi/utils.hpp

include/scalfmm/tools/data_generate.hpp

include/scalfmm/tools/fma_dist_loader.hpp

include/scalfmm/tools/vtk_writer.hpp