Add support for exchanging ghost cells

[jipolanco]

Add support for exchanging neighbouring data between MPI processes.

• The user should be able to select the number of planes to exchange along each decomposed dimension.
• In principle, the number of exchanged planes may be different for each dimension.
• To make the implementation easy, only support exchanges with a single process along each dimension (limiting the maximum possible number of exchanged planes). This may be extended in the future.

[chowland]

Hi @jipolanco, just thought I would let you know that I am starting to work on this in a new repo HaloPencils. I'm using the approach of 2decomp as a guide, since that is what I'm most familiar with.
I'm at a very early stage, and progress may be slow as this is not my main research focus, but if you think this is a useful implementation, then I would be happy to consider merging this into PencilArrays once it's ready for use.

[jipolanco]

Looks great! I'll be very happy to include this in PencilArrays once this is ready to use. Let me know if I can be of any help.