Update setup.py

Author: gduscher

setup.py

@@ -68,6 +68,7 @@
     include_package_data=True,
     # https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-dependencies
     extras_require={
+        'MPI': ["mpi4py"]
     },
     # If there are data files included in your packages that need to be
     # installed, specify them here.  If using Python 2.6 or less, then these