added py-mpi4py as dependency

packages/py-ssg/package.py

@@ -38,6 +38,7 @@ class PySsg(PythonPackage):
 
     version('master',  git="https://xgitlab.cels.anl.gov/sds/py-ssg.git")
 
-    depends_on('ssg')
+    depends_on('ssg+mpi')
+    depends_on('py-mpi4py')
     depends_on('py-margo')
     depends_on('py-pkgconfig')