Missed stacks when stacking in MPI mode

[marlena6]

Sometimes not all stacks are run when running stacking_pipeline.py with mpirun.
Good to check that the number of pks is what is expected.
If not, going back and running it out of MPI mode does the trick.
Still, need to figure out what's going wrong here.

[marlena6]

Note: I think this happens when the zmin or zmax is wrong, causing at least one of the processes to terminate. I should add a warning message about this.