MPI refactoring

See merge request !82

MPI refactoring: replaced DimsCreate and CreateNewGroup in MpiCommunicator with plain function wrappers

Also from DistributedMeshSynchronizer, PVTUReader and PVTUWriter

This affects DistributedArray, DistributedArrayView, DistributedVector,
DistributedVectorView and DistributedMatrix. Allocators were added to
DistributedArray and DistributedVector. Also updated all benchmarks and
unit tests.

MPI refactoring: removed MpiCommunicator from DistributedNDArray, added Allocator parameter to NDArray

This allows the user to set the required thread level. Initializing MPI
with threading support is needed when MPI functions are called from
multiple threads. All common MPI libraries seem to provide this feature.