MPI: removed default values for MPI_Comm parameters and moved NullGroup checking outside HAVE_MPI

- The "NullGroup" should not be used even when built without MPI,
  otherwise the behaviour is very bug-prone because "NullGroup" usage is
  not caught and changing the build type leads to a different semantics.
- "AllGroup" is not a good default value for the parameters, considering
  that the class attributes are initialized to "NullGroup".