Allow CellCenteredFlux to work for mixed systems

[kidder]

Previously CellCenteredFlux assumed that the flux_variables were the same as the evolved variables.

Proposed changes

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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.

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