Sink MPI part 1: engine, proxies and strays

This MR splits 1975 to focus on all except the tasks. More precisely, it adds MPI logic between engines on different nodes, such as the proxies, the strays, and the space-rebuilding steps. It's quite a substantial technical merge request.

I found and fixed some inconsistencies and bugs for the sinks. I also found some bugs for the BHs; I left comments throughout the code.

TODO:

• Clean debugging info in proxy.c
• Discuss about adding here or in another MR some MPI doc (see my comments in the code)
• Add a debugging flag to disable all sink interactions and only allow drift, kick and gravity --> testing purpose to ensure the sink is propagated correctly from one MPI domain to the next
• Add an example with sinks in the ICs + initial velocity to test this behaviour
• Tell Matthieu to add this example in the testing suite