Executable for MC: initialize background

[ffoucart]

Proposed changes

Create base MC executable

Upgrade instructions

None so far

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]

Work in progress -- first commit creates the background fluid and metric variables, EoS and neutrino table.

[ffoucart]

@nilsdeppe Not sure if we want one big PR for the MC code, or to commit partial work once we have something useful. I'm keeping the work we have done so far (slightly cleaned up for compatibility with other executables) here.

[ffoucart]

Rebased on develop to fix minor conflict with RunEventsAndTriggers (template change, option renamed)

[ffoucart]

Rebased to make dependent on #6424 . The first commit is now from that PR, the second commit creates the executable; the final part to make it work with input files is in #6403