Remove All but pFUnit from the default branch

[connoraird]

Definition of Done / Acceptance Criteria

Instead of covering three frameworks, we will focus on just pFUnit as this is the framework which works with MPI.

• Store the current state in an "archived" branch
• Within main, remove all reference to veggies and test-drive

Testing

• Veggies and test-drive should also be removed from the CI tests
• All remaining tests still pass

Documentation

• Do the same on the lesson
• Update READMEs to remove veggies and test-drive