Add multi-rank domain decomposition test

[jeff-cole]

This test case extends read_domain_resample to use multiple processors and domain decomposition to resample the data.

File shared_testing.py was copied from mo-marqh/tcd-XIOS-demonstration branch mpirankborder

[mo-marqh]

Hi Jeff

this is failing the CI testing, it's trying, but it's falling over trying to allocate threads on the Ubuntu Action runner.
It's only a little VM

there's a flag to help with this:
https://github.com/MetOffice/tcd-XIOS-demonstration/pull/13/files#diff-42afd176f6b281857ba3050c03b8f51fe50437203375902a27fb88861ad6c40aR38

but it needs conditionally activating, for openmpi cases only

you'll need to add an environment variable into .github/workflows/buildTest.yml
that activates this trigger, but only on github, as we don't want nor need it on other platforms, with different MPI implementations

https://github.com/MetOffice/tcd-XIOS-demonstration/pull/12/files#diff-c66e53f9783d6581f7b6a720487764481cd7a10fde3d4d3a235f3390b6749cce

that should at least get this test rerun get further

many thanks
mark

[jeff-cole]

Thanks, that worked.

[mo-marqh]

many thanks @jeff-cole