Merged requested to merge cell_graph into master
I think we are missing a tool to analyze the tree. Here is one: https://obswww.unige.ch/~lhausamm/swift/cell.html
Current MPI implementation writes a file per rank during the simulation and then uses a bash script to merge them together.
There is still much work to be done (such as finding easily the neighbors, but I will not have time to do it before a long time).