WIP: Allow ranks to be empty on restart, if that was true at last save

Possible fix for #735 (closed).

added bug enhancement labels

Not tested yet, as hard to get a solution with missing ranks. Only other thought is that this seems like a lot of effort for an unlikely occurrence, maybe we should just change the error report into a message and hope the user sees it. I suspect if this fails many other issues might also be present.

Thanks @pdraper. This happened in @rttw52's large run. I don't know whether he has a small reproducible example to test it.

I am now very confused.

I am running the IsolatedGalaxy_feedback example with 16 MPI ranks. This will lead to silly imabalance and indeed the code redistributes after 2 steps. It then also decides to leave a few nodes out. There is a check in place preventing this from happening. So first problem: how did we get there in the first place in the sibelius run? Then, disabling the check and running further, I force a restart dump. Then I resume the run and everything goes smoothly. No complaints. All this in master.

So, I must have misunderstood the original problem that led to #735 (closed). Or there is something else at play here.

closed