Optically thick sphere

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Proposed changes

Example of a full MC evolution, not coupled to the fluid

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.

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Provided as an example of code usage so far (though the test itself is useful as well).
Depends on 5902 and 5907.

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Rebased against the latest MC changes.

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@SamanthaRath This is the current test set up for optically thick sphere and boosted sphere. I just rebased on develop and recompiled; seems to work.