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.

include/xgboost/metric.h

@@ -29,9 +29,9 @@ class Metric {
    *        the average statistics across all the node,
    *        this is only supported by some metrics
    */
-  virtual float Eval(const std::vector<float>& preds,
-                     const MetaInfo& info,
-                     bool distributed) const = 0;
+  virtual bst_float Eval(const std::vector<bst_float>& preds,
+                         const MetaInfo& info,
+                         bool distributed) const = 0;
   /*! \return name of metric */
   virtual const char* Name() const = 0;
   /*! \brief virtual destructor */