Cell graph

I think we are missing a tool to analyze the tree. Here is one: https://obswww.unige.ch/~lhausamm/swift/cell.html

The idea is to simply dump in a csv file the information that we need and then use HTML/javascript to display everything.

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).