Add a mpi implementation of the task graph

In the previous implementation, we were considering only the dependencies in the master MPI rank. Here I changed the format a bit and created a new script to generate the figure.

For the output, I am using the csv format and write 1 file per rank.

With the SedovBlast_3D example, it takes about 10ms.

My new python script is reading all the files and merge them together to produce a dot file. At the end, I have a system call to transform the dot file to png. You can still use the script as before by providing a single file (e.g. ./plot_tasks_dependencies.py dependency_graph_0000.csv).