Merge branch 'cineca/mpi' into develop

* cineca/mpi:
  Added specialization of MPIDataType for bool
  Added missing operations to MpiDefs.h
  Added assertions to MpiCommunicator.h checking that MPI functions are not called with the null group
  Whitespace and typo fixes in MpiCommunicator
  Fixed the TNL_MPI_PRINT macro
  Fixed signature of the Allreduce method of communicators
  Added NullGroup to communicators because it is useful for initializations
  Removed empty file and directory in UnitTests