feature: Treat a "serial" distribution the same as a single partition

[twsearle]

Description

The "serial" decomposition generates a full copy of the atlas mesh on each MPI process. The boundary/periodic conditions should be updated in the same way as for a single MPI process in this case. This change bypasses later changes to the remote indices that would be made during atlas::mesh::actions::build_parallel_fields, preserving the setup that is used for a single processor to guarantee the correct swaps at the periodic boundaries and the north fold.

Impact

This should fix issues at the periodic boundaries of all ORCA grids for nparts >1 to match nparts == 1 when serial decomposition is used. However, there is still a little work to do to fix cases of checkerboard and equal_regions partitioners.

This is a requirement to hit operational performance benchmarks in the JEDI-based Observation Processing Application (JOPA) at the Met Office.

Testing

• addition of a new ctest in test_orca_mesh_boundaries. For now, this only tests this one case of 2 MPI processes and the serial distribution, but in the future I would like to add tests for other distributions too
• tested at the Met Office as a component in JOPA

[FussyDuck]

All committers have signed the CLA.