Monte carlo packets

[ffoucart]

Proposed changes

Add packets for MC evolutions, and ability to evolve packets along a geodesic

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

Very preliminary; packets cannot at this point be included in actual evolution...

[kidder]

you are missing MonteCarlo in this list

Better is to update this to the following (which is what we are slowly doing to all test directories):

• Change line 14 to just be an empty list ""
• Add the following to the end of the file:

target_link_libraries(
  ${LIBRARY}
  PRIVATE
  Monte Carlo
  )

[ffoucart]

@kidder : The code for the evolution of a single packet for a time step along a geodesic seems to pass the standard tests now. I'm not sure what a more appropriate test would be here; the one I implemented is obviously simplistic. I could write a python function reproducing the whole second-order time stepping, and compare results with random inputs, I guess? It's not clear to me whether we will actually perform time steps using this method, or incorporate the time-stepping within another part of the code once we try to actually move packets on a grid.

Apart from that, I think this PR is ready for review (and, obviously, fairly low priority...).

[nilsdeppe]

Looks good! A few small comments you can squash right away when you rebase

[ffoucart]

@nilsdeppe More changes than I'd have liked in order to match the latest clang-tidy requirements, but this should be ready for you again. Note that I needed to add a default constructor to the packet struct -- clang-tidy was unhappy with packet creation that did not explicitly set the contents of the packet.

[ffoucart]

(The failing test is some paraview thing... I'm guessing this has nothing to do with this PR)

[nilsdeppe]

CI failure is some unrelated paraview thing.