Cudf support. (#4745)

* Initial support for cudf integration.

* Add two C APIs for consuming data and metainfo.

* Add CopyFrom for SimpleCSRSource as a generic function to consume the data.

* Add FromDeviceColumnar for consuming device data.

* Add new MetaInfo::SetInfo for consuming label, weight etc.

tests/python/testing.py

@@ -1,5 +1,6 @@
 # coding: utf-8
-from xgboost.compat import SKLEARN_INSTALLED, PANDAS_INSTALLED, DT_INSTALLED, DASK_INSTALLED
+from xgboost.compat import SKLEARN_INSTALLED, PANDAS_INSTALLED, DT_INSTALLED
+from xgboost.compat import CUDF_INSTALLED, DASK_INSTALLED
 
 
 def no_sklearn():
@@ -31,3 +32,8 @@ def no_matplotlib():
     except ImportError:
         return {'condition': True,
                 'reason': reason}
+
+
+def no_cudf():
+    return {'condition': not CUDF_INSTALLED,
+            'reason': 'CUDF is not installed'}