MPI cleanup removes useful traceback from output

[msyriac]

These lines:

pixell/pixell/mpi.py

Line 23 in d3167f9

def cleanup(type, value, traceback):

when not commented cause the MPI output to not print the traceback for the error that caused the script to crash (if any), which leads to a lot of wasted time. I can reproduce this on niagara with both Intel MPI and OpenMPI.