fix(mpiarray): use non-pickling variant of MPI.Comm.Allreduce with MPIArrays
fix(mpiarray): use non-pickling variant of MPI.Comm.Allreduce with MPIArrays

Pickling can result in MPIArrays that are missing important attributes
(such as .axis).

Important for https://github.com/radiocosmology/caput/pull/162

radiocosmology/draco,radiocosmology/draco