Make mpi_helper's method non-rooted if root=None

[Baharis]

A common pattern in multiprocessing is to collect data on the root (typically rank 0), process it there, and broadcast it back to other ranks. However, the better practice is typically to use a non-rooted operation: pass the inputs between all ranks and perform calculations independently on each. This removes the need for a second communication (broadcast) and improves code readability.

Non-rooted approach is inferior only in these rare cases where the cost of the switching to non-rooted operation is much larger than the cost of the secondary broadcast. Non-rooted approach might potentially also cause issues with memory consumption if the input or temporary objects (created when analyzing input) are very volumous.

In xfel, communicating between ranks can be done using the mpi_helper class, whose methods alias more complex operations to improve readability and remove repetition. Currently all of the mpi_helper methods are rooted, forcing one to copy or rewrite them whenever an non-rooted collective is more desired.

This patch slightly modifies all mpi_helper methods, allowing them to use non-rooted mpi4py collectives. Whenever their keyword argument root (default 0) is set to None instead of an integer, the methods will use a non-rooted collective instead of the collective variant. This will produce identical output on all ranks, removing need for some follow-up broadcasts and allowing for cleaner and potentially faster code.

Following the update, I have also modified three (cctbx_project) instances where mpi_helper methods were used and could be changed to utilize non-rooted collective variants to potentially speed up the execution. Each was done in a separate commit to revert if change is not desired. The change will also positively affect code in other repositories utilizing the mpi_helper.

[phyy-nx]

Thanks, the mm24 change would have used up too much memory I think, but the other changes look great!