Add possibility to pass comm to MPI classes.
Signed-off-by: Andre Klein <email@example.com>
@kleina Please check my comments and if nothing stands against it: 1) spawn a task to cover it. 2) cover it 😉 .
I just saw that the default option is already implemented. Then I would just remove it in the playground or explain that both ways are possible. Because otherwise people might think that they always have to do it.