Allow use of default MPI Communicator with Fortran API

[andrewcoughtrie]

Although Vernier will use a default MPI communicator when called from the C++ API this has not been passed through to the Fortran API. Have added the capability here.

I haven't been able to get the testing working using pFUnit, I'm not sure why, for some reason it hangs. I do know that pFUnit controls the MPI communicator and duplicates it, it does state in the source not to use MPI_COMM_WORLD outside the main pFUnit program MpiContext.F90#L65 so perhaps they are doing some thing that doesn't play well if we try to use MPI_COMM_WORLD for Vernier as well.

As it is I've created a simple system test to check the call without an argument works, we perhaps need a better mechanism for testing this at some point.

I've also done a bit of tidying up of all the test names.

[mo-mglover]

Looks good to me. I agree that the optional argument approach is the right way to do this.

I've made a couple of inline comments.
[Noticed a few capitalisation issues, too, but I haven't nit-picked about those!]

Also noted, just for the papertrail, that there's a number of other bits of tidying up mixed in, such as a cmake function to facilitate adding new Fortran pfunit tests and removal of trailing whitespace.