Suggestion to avoid recursive mpirun calls

[danielwolverson]

Hi - many thanks for sharing this code, which looks extremely useful! Just to let you know, I've had a problem with phonon_sort, which calls matdyn: this generates a recursive mpirun call which is forbidden on my cluster, so crashes. I've compiled a local, serial copy of matdyn and edited phonon_sort to remove the use of mpirun - I'm posting this in case it might help others if they have the same problem.