Add basic unittests for gpu-hist method. (#3785)

* Split building histogram into separated class.
* Extract `InitCompressedRow` definition.
* Basic tests for gpu-hist.
* Document the code more verbosely.
* Removed `HistCutUnit`.
* Removed some duplicated copies in `GPUHistMaker`.
* Implement LCG and use it in tests.

include/xgboost/data.h

@@ -151,10 +151,11 @@ struct Entry {
 };
 
 /*!
- * \brief in-memory storage unit of sparse batch
+ * \brief In-memory storage unit of sparse batch, stored in CSR format.
  */
 class SparsePage {
  public:
+  // Offset for each row.
   HostDeviceVector<size_t> offset;
   /*! \brief the data of the segments */
   HostDeviceVector<Entry> data;