Merge pull request #81 from JCGoran/fix_mpi4py

Add fix for mpi4py version 4

Author: mdjurfeldt

pymusic/mpi_compat.h

@@ -4,3 +4,8 @@
 typedef void *PyMPI_MPI_Message;
 #define MPI_Message PyMPI_MPI_Message
 #endif
+
+#if (MPI_VERSION < 4) && !defined(PyMPI_HAVE_MPI_Session)
+typedef void *PyMPI_MPI_Session;
+#define MPI_Session PyMPI_MPI_Session
+#endif