Add pytools.mpi: Runs a callable with a given number of MPI ranks.

pytools/mpi.py

+def run_with_mpi_ranks(py_script, ranks, callable, *args, **kwargs):
+    import os
+    if "BOOSTMPI_RUN_WITHIN_MPI" in os.environ:
+        callable(*args, **kwargs)
+    else:
+        import sys
+        newenv = os.environ.copy()
+        newenv["BOOSTMPI_RUN_WITHIN_MPI"] = "1"
+
+        from subprocess import check_call
+        check_call(["mpirun", "-np", str(ranks), 
+            sys.executable, py_script], env=newenv)
+