Ghost zone comm mc sum fluid coupling

[ffoucart]

Proposed changes

Add a mutator responsible for summing the contributions to the coupling between fluid and Monte-Carlo gathered from ghost zones to the values stored locally from the evolution of MC packets in live zones.

Upgrade instructions

NA

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

Depends on #6550

[ffoucart]

The first commit of this PR is just #6550 . Only the second commit is relevant. There is a minor change to the test that checks work from 6550, but as the test didn't catch any error, i just kept it in this PR.

[kidder]

why isn't this just index_3d[dimension]?

[ffoucart]

I added a much longer comment. I assume that the difference with what you imagine is that we are copying from the ghost zones of the neighbor onto the live points of the current mesh (rather than the other way around, usually), and all meshes used here include ghost points in their indexing.

[kidder]

ah, thanks, the comment helps a lot

[ffoucart]

Updated after rebasing on merged PR #6550 and adding a comment to address Larry's question.