Mc optically thick sphere

[ffoucart]

Proposed changes

Add initial conditions for a homogeneous sphere (fluid variables only determined by whether we are inside or outside the sphere)

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]

Depends on #6355 (first five commits are from that PR)