[Warnings] Fix MPI 1 deprecation warnings (by switching to MPI 2).

This fixes warnings of the following form about functions deprecated in MPI 2.
```
/home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/parallelamgtest.cc: In function ‘int main(int, char**)’:
/home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/parallelamgtest.cc:217:3: warning: ‘int MPI_Errhandler_create(void (*)(ompi_communicator_t**, int*, ...), ompi_errhandler_t**)’ is deprecated: MPI_Errhandler_create is superseded by MPI_Comm_create_errhandler in MPI-2.0 [-Wdeprecated-declarations]
   MPI_Errhandler_create(MPI_err_handler, &handler);
   ^~~~~~~~~~~~~~~~~~~~~
In file included from /home/joe/Projekte/dune-simd/dune-common/dune/common/parallel/mpitraits.hh:23:0,
                 from /home/joe/Projekte/dune-simd/dune-common/dune/common/parallel/plocalindex.hh:9,
                 from /home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/anisotropic.hh:8,
                 from /home/joe/Projekte/dune-simd/dune-istl/dune/istl/paamg/test/parallelamgtest.cc:7:
/usr/lib/x86_64-linux-gnu/openmpi/include/mpi.h:1345:20: note: declared here
 OMPI_DECLSPEC  int MPI_Errhandler_create(MPI_Handler_function *function,
                    ^~~~~~~~~~~~~~~~~~~~~
```

dune/istl/paamg/test/parallelamgtest.cc

@@ -214,8 +214,8 @@ int main(int argc, char** argv)
 {
   MPI_Init(&argc, &argv);
   MPI_Errhandler handler;
-  MPI_Errhandler_create(MPI_err_handler, &handler);
-  MPI_Errhandler_set(MPI_COMM_WORLD, handler);
+  MPI_Comm_create_errhandler(MPI_err_handler, &handler);
+  MPI_Comm_set_errhandler(MPI_COMM_WORLD, handler);
 
   int N=100;
 

dune/istl/test/matrixredisttest.cc

@@ -120,8 +120,8 @@ int main(int argc, char** argv)
 {
   MPI_Init(&argc, &argv);
   MPI_Errhandler handler;
-  MPI_Errhandler_create(MPI_err_handler, &handler);
-  MPI_Errhandler_set(MPI_COMM_WORLD, handler);
+  MPI_Comm_create_errhandler(MPI_err_handler, &handler);
+  MPI_Comm_set_errhandler(MPI_COMM_WORLD, handler);
   int procs;
   MPI_Comm_size(MPI_COMM_WORLD, &procs);