[DOC] Group by aggregate using linear algebra usage (#1251)
[DOC] Group by aggregate using linear algebra usage (#1251)

* Reusing code from the SystemML tutorial KDD 2017
apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml,apache/incubator-systemml