This example is identical to the example contained in ../1d/, except it
demonstrates how to use MPI to compute spectra in parallel. Run this
example by executing:
$ S4 binary_grating_mpi.lua
The script will produce as many output files as there are MPI nodes,
named out0.txt, out1.txt, out2.txt, etc. The concatenation of these
files should be similar to ../1d/binary_grating.txt
