Mc optically thick sphere

[ffoucart]

Proposed changes

Provide a first initial condition for the standalone MC code -- a uniform sphere with different fluid properties inside and outside the sphere.

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

[ffoucart]

Depends on #6355 (the first five commits are from that PR).
@nilsdeppe @kidder : This is the next step for the MC executable -- initial data that does not directly rely on the GrMhd solutions.