Add MPI tutorial

[joaander]

Description

Add a tutorial on how to execute MPI simulations with HOOMD-blue.

Resolves: #42

Checklist:

• [x] I have reviewed the Contributor Guidelines.
• [x] I agree with the terms of the HOOMD-blue Contributor Agreement.
• [x] My name is on the list of authors.

Questions for reviewers

Reviewers, please answer these questions in addition to any specific line-level comments you may have.

• Does the tutorial help you understand how to use HOOMD-blue to run simulations with MPI parallelization?
• Does the tutorial miss any important concepts? If yes, which ones?
• What concepts, if any, does the tutorial cover in too much detail for novice HOOMD users? What part do you think was too much?
• What concepts, if any, does the tutorial not cover with sufficient detail? What details need to be added?
• Is there anywhere that the tutorial uses fluid representations (using different words to describe the same thing)?

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[lyrivera]

I have added myself as a reviewer.

[joaander]

All, thanks for the great comments. This is ready for another round of review.

[joaander]

TODO:

• [x] move MPI tutorial before the custom action tutorial.

[joaander]

Thanks all for the feedback! I am going to merge this and include it in the beta 9 release.