Proposed changes

• Makes M1Grey solutions option creatable to be consistent with other code

Upgrade instructions

• New AnalyticSolutions for M1Grey should be added to RadiationTransport::M1Grey::Solutions::all_solutions
• Executable was renamed to EvolveM1Grey
• Input file needs updating (see tests/InputFiles/RadiationTransport/M1Grey/ConstantM1.yaml)

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