Bh communication fixes

Two black hole fixes over MPI:

• Do not activate MPI communications that have no relation to the BH code.
• Add the dependency that should have been there between particles and BHs.

Given that the broken code was apparently working in the current master, I am not sure how to test it.

Agreed. What problem turned these bugs up?

Some on-going work with the BH swallowing gas particles code. The issue here would be unnoticed as the tasks we try to unskip always exist in the test cases we have tried until now.

I'll accept, since you are more likely to notice the problem.

merged

mentioned in commit 6225c0db