## Background 
This PR is part of #120, where the basis of strategy for MPI
parallelisation is described.

## Change description
As all thermodynamics operations are local to a grid cell all required
MPI communication, is handled by NetCDF4 library. Therefore the only
required steps are:

- [x] Initialize and finalize MPI stack
- [x] Read decomposition metadata on each rank
- [x] Read and write the necessary part of the grid on each rank
(depends on #330)
- [x] Tests for parallel I/O 

**NOTE** PR #432 should be merged into this branch before it is merged
into develop

To run `run_simple_example.sh` you will need to generate the following
netcdf file

```
$ ncdump partition.nc 
netcdf partition {
dimensions:
        P = 1 ;
        L = 1 ;
        globalX = 30 ;
        globalY = 30 ;

group: bounding_boxes {
  variables:
        int global_x(P) ;
        int global_y(P) ;
        int local_extent_x(P) ;
        int local_extent_y(P) ;
  data:

   global_x = 0 ;

   global_y = 0 ;

   local_extent_x = 30 ;

   local_extent_y = 30 ;
  } // group bounding_boxes
}

```

# Add MPI support to ParaGrid
closes #534
closes #454
closes #448 

---
# Task List
- [x] modify `make_init_para24x30.py` to use new DG data layout and
remove CG vars
- [x] add MPI doctest support to `ParaGrid_test.cpp`
- [x] move `extent` and `start` variables into `ModelArray`'s
`DimensionSpec`
- [x] add special MPI test case to `ParaGrid_test.cpp`
- [x] add MPI support to `getModelState`
- [x] add MPI support to `dumpModelState`
- [x] add MPI support to `writeDiagnosticTime`
- [x] reinstate test `ConfigOutput_test.cpp` for MPI builds
- [x] add MPI support to `ERA5`/`TOPAZ` tests

---
# Change Description

After #331 added MPI parallelisation for thermodynamics on the RectGrid,
this PR does the same for the Parametric grid. This should then check
off the second task in #120 (_"MPI parallelization of thermodynamics
where all operations, except for I/O, are local to an MPI rank"_)

---
# Test Description

- `ParaGrid_test.cpp` tests core functionality of ParaGrid (serial and
MPI)
- `./nextsim --config-file config_para24x30.cfg` should provide an
integration test (serial and MPI) (based on #506)

---
# Further work (for a future PR)

- add MPI to dynamics (to close PR #120)
- implement halo comms
- implement boundary conditions (as part of MPI) 
- tidy naming of variables in Domain Decomp tool