Typos to fix

[connoraird]

• Not - > Note, Writing your first unit test
• ep 4 lesson, constructors, this should be testParameter should be this
• ep 4 exercise,
  • in the challenge the calls to the src code is incorrect (i.e. input params in the wrong order)
  • test_find_steady_state.pf should not exist
• ep 6, neighbours should defualt ot MPI_PROC_NULL not -2