Add exercise for converting standard Fortran unit test into pFUnit

[connoraird]

Definition of Done / Acceptance Criteria

• Using the temperatures example, add an exercise for converting the "good" unit test written in standard fortran into a pFUnit test.

Testing

• A new CI test is added for this exercises

Documentation

• A README is added which covers...
  • The task to be completed
  • How to build the code
  • Details about the solution

Dependencies

#27