Pass around neighbor meshes when determining AMR decisions

[kidder]

Proposed changes

In addition to passing an Element's AMR decisions, pass along what its new Mesh will be.
This will enable the mortar mesh to be initialized when adjusting the Domain during AMR.

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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