Use bst_float consistently throughout (#1824)

* Fix various typos

* Add override to functions that are overridden

gcc gives warnings about functions that are being overridden by not
being marked as oveirridden. This fixes it.

* Use bst_float consistently

Use bst_float for all the variables that involve weight,
leaf value, gradient, hessian, gain, loss_chg, predictions,
base_margin, feature values.

In some cases, when due to additions and so on the value can
take a larger value, double is used.

This ensures that type conversions are minimal and reduces loss of
precision.

src/data/sparse_page_dmatrix.h

@@ -48,7 +48,7 @@ class SparsePageDMatrix : public DMatrix {
     return buffered_rowset_;
   }
 
-  size_t GetColSize(size_t cidx) const {
+  size_t GetColSize(size_t cidx) const override {
     return col_size_[cidx];
   }
 
@@ -111,7 +111,7 @@ class SparsePageDMatrix : public DMatrix {
     std::vector<SparseBatch::Inst> col_data_;
   };
   /*!
-   * \brief Try to intitialize column data.
+   * \brief Try to initialize column data.
    * \return true if data already exists, false if they do not.
    */
   bool TryInitColData();