Write ELLPACK pages to disk (#4879)

* add ellpack source
* add batch param
* extract function to parse cache info
* construct ellpack info separately
* push batch to ellpack page
* write ellpack page.
* make sparse page source reusable

src/data/ellpack_page.cc

@@ -1,18 +1,16 @@
 /*!
  * Copyright 2019 XGBoost contributors
- *
- * \file ellpack_page.cc
  */
 #ifndef XGBOOST_USE_CUDA
 
 #include <xgboost/data.h>
 
-// dummy implementation of ELlpackPage in case CUDA is not used
+// dummy implementation of EllpackPage in case CUDA is not used
 namespace xgboost {
 
 class EllpackPageImpl {};
 
-EllpackPage::EllpackPage(DMatrix* dmat) {
+EllpackPage::EllpackPage(DMatrix* dmat, const BatchParam& param) {
   LOG(FATAL) << "Internal Error: XGBoost is not compiled with CUDA but EllpackPage is required";
 }