Export Python Interface for external memory. (#7070)

* Add Python iterator interface.
* Add tests.
* Add demo.
* Add documents.
* Handle empty dataset.

demo/c-api/external-memory/README.md

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+Defining a Custom Data Iterator to Load Data from External Memory
+=================================================================
+
+A simple demo for using custom data iterator with XGBoost.  The feature is still
+**experimental** and not ready for production use.  If you are not familiar with C API,
+please read its introduction in our tutorials and visit the basic demo first.
+
+Defining Data Iterator
+----------------------
+
+In the example, we define a custom data iterator with 2 methods: `reset` and `next`.  The
+`next` method passes data into XGBoost and tells XGBoost whether the iterator has reached
+its end, and the `reset` method resets iterations. One important detail when using the C
+API for data iterator is users need to make sure that the data passed into `next` method
+must be kept in memory until the next iteration or `reset` is called.  The external memory
+DMatrix is not limited to training, but also valid for other features like prediction.