Replace MPI_Allreduce with allreduce

@frey_m suggested in #485 (comment 18514) to use allreduce instead of MPI_Allreduce:

Please consider also the use of Datatypes.h with one of the allreduce functions.

[..] Here is an example.

changed milestone to %OPAL 2.4.0

added Enhancement label

mentioned in issue #485 (closed)

The following files still have MPI_Allreduce calls:

• ippl/src/Particle/ParticleSpatialLayout.h
• src/Algorithms/ParallelSliceTracker.cpp
• src/Algorithms/bet/EnvelopeBunch.cpp
• src/Amr/BoxLibLayout.hpp

created merge request !322 (merged) to address this issue

mentioned in merge request !322 (merged)

assigned to @frey_m

closed via merge request !322 (merged)

mentioned in commit e574dc13