Add MPI example

[ArangoGutierrez]

Add a walkthrough MPI example

[syl-ilumb]

Thank you for adding an excellent MPI example, @ArangoGutierrez!

I have the following feedback:

• In the line $ sudo singularity exec -w centos-7.sif yum groupinstall -y "Development Tools" remove .sif

• The command singularity exec centos-7 mpicc openmpi-2.1.0/examples/ring_c.c -o ring is being issued from the openmpi-2.1.0 directory - as you suggest a cd after it ...

• For consistency, sudo singularity build centos7.sif centos-7 requires a “-” in the name of the SIF container

• Can you please add a statement re: any restrictions re: MPI on the host - e.g., implementation (MPICH vs. OpenMPI), versions, etc.

• FYI: I needed to increase the size of my VM to 2 GB in order to build MPI

-Ian