MPI_CXX_COMPILE_FLAGS is used as it is in compilation flags while it is a list (contains semicolons) so that we remove the semicolons to prevent errors

Fix bug with optidis. The level in rootcell should be set in the construcor (needed in M2M operator)

Use inastemp

See merge request solverstack/ScalFMM!15

Remove all warnings with gcc 8: memset et memcopy une now reinterpret_cay<char*> and we remove restrict fir target and source in p2P