Document MPI

[mattwelborn]

Is your feature request related to a problem? Please describe.
MPI is supported but not documented.

Describe the solution you'd like
There should be documentation for:

• running MPI with compute
• running MPI with compute_procedure
• developing MPI-aware program harnesses

Additional context
This issue arose during discussions with https://github.com/MRChemSoft/mrchem

[dgasmith]

@WardLT is this ready to be documented and supported?

[WardLT]

Yes, and I can make documentation for how the MPI-ready Program harnesses function.

Where in the docs should it go? Programs or Developer Documentation?

[dgasmith]

I think a new page under "User Interface" since this is general to all program with a note in the environment detection page that MPI isn't automatically found.

[WardLT]

Thanks. I can do that