Add tags for ghost zone communication in MC

[ffoucart]

Proposed changes

Add tags and structures to hold ghost zone data for MC.

Upgrade instructions

None

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.

[ffoucart]

@nilsdeppe @kidder : rough draft of the first structures needed for GZ communication in MC... if this is not what you had in mind, let me know.