Reduce number of all-to-all communications

Attempt to answer #273 (closed). Just a place marker for the moment.

Added 1 commit:

• bad02c5f - Also reduce rebuild at same time as updates and timesteps

mentioned in issue #273 (closed)

Added 1 commit:

• c0d331e7 - Introduce a collectgroup API to handle the group of values we want to MPI reduce at the same time

Added 63 commits:

• c0d331e7...ab6d8549 - 62 commits from branch master
• 9550d87d - Merge branch 'master' into mpi-less

Added 72 commits:

• 9550d87d...bd9b222b - 71 commits from branch master
• c073167f - Merge branch 'master' into mpi-less

Title changed from WIP: Reduce number of all-to-all communications to Reduce number of all-to-all communications

@nnrw56 probably time for you to have a look at this as well.

LGTM!

I will wait until we solved the MPI+gravity bug before merging this in.

Should be good to go now.

Yes, perfect, thanks.

Status changed to merged

mentioned in commit b90e24d0