Reuse of comm buffers across containers

[pgrete]

Following a discussion along

          We don't have any control over that and I agree that if you had a task list that had independent groups of tasks that worked on different containers but also communicated you could possibly run into issues where one container grabs messages meant for another container. That is similar to some of the issues I had to fix for multi-grid in this PR. I am not sure what the best approach to dealing with this is. Making buffers for each container doesn't seem ideal and it could be a little tricky to build buffers and MPI tag maps for multiple containers in the current framework (since now buffers are built right after remesh, not at the point when containers are first asked for). Maybe we should put up a warning issue about this.

Originally posted by @lroberts36 in #1143 (comment)

it might be worth to think about how to handle this situation.
One potential way is see is to add a "requres_comm` flag (for comm id) to tasks so that in the task list we can check that only tasks with the same flag/id are processed within a single region.