Fix Vernier

Vernier support is broken now that Vernier requires a communicator in its `init` call :( The communicator needs to be taken for LFRic from mpi_mod, e.g.:
```
type(lfric_comm_type) :: comm
comm = modeldb%mpi%get_comm()
call vernier_init(comm%get_comm_mpi_val())

```

It would be good if this piece of code could be automatically created - ie identify in the main program where to insert it etc etc

There is also an ongoing discussion about vernier, additional work might be required.

Start at the generated LFRic main program and the Vernier integration, then inspect how mpi_mod exposes modeldb%mpi%get_comm() and how vernier_init is called. The work is done when Vernier receives the LFRic MPI communicator through generated code and the ongoing Vernier discussion's additional requirements are addressed.

Written by the indexing model from the issue text.