Add unit tests for MPI parallel imaging

[israelmcmc]

In particular, for the classes DataIF_Parallel and ParallelImageDeconvolution.

They probably need to be added in a different CI pipeline, since you need to run in an environment with the MPI backend installed

See discussion around #274 (comment)

[augustus-thomas]

In principle we can change our CI/CD tool to install openmpi. We could add to the Install step in the GitHub workflows here to also run

sudo apt install openmpi-bin openmpi-common libopenmpi-dev 

We run a similar package install in our docs workflow here. This works because codecov is using an Ubuntu system. I am not sure if the above are the correct packages needed by mpi4py, or if some of them are not needed.

[israelmcmc]

Yeah, that sounds good @augustus-thomas. It would be good though to run the units test in two build systems, with and without MPI, since we won't require every user to install it. I've never done it, but I think codecov handles this kind of things well: https://docs.codecov.com/docs/merging-reports