MPI refactoring: split MpiCommunicator into plain functions in the TNL::MPI namespace

src/Benchmarks/DistSpMV/tnl-benchmark-distributed-spmv.h

src/Benchmarks/LinearSolvers/tnl-benchmark-linear-solvers.h

src/Benchmarks/ODESolvers/tnl-benchmark-ode-solvers.h

src/TNL/Communicators/MpiCommunicator.h

src/TNL/Containers/DistributedArray.hpp

src/TNL/Containers/Expressions/DistributedComparison.h

...NL/Containers/Expressions/DistributedVerticalOperations.h

src/TNL/MPI.h

src/TNL/MPI/DummyDefs.h

src/TNL/MPI/Print.h

src/TNL/MPI/ScopedInitializer.h

src/TNL/MPI/Utils.h

src/TNL/MPI/Wrappers.h

src/TNL/MPI/getDataType.h

src/TNL/MPI/selectGPU.h

src/TNL/Meshes/DistributedMeshes/BufferEntitiesHelper.h

...Meshes/DistributedMeshes/DistributedGridIO_MeshFunction.h

...NL/Meshes/DistributedMeshes/DistributedGridSynchronizer.h

src/TNL/Solvers/Solver_impl.h

src/Tools/tnl-game-of-life.cpp