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.

demo/distributed-training/plot_model.ipynb

@@ -6,7 +6,7 @@
    "source": [
     "# XGBoost Model Analysis\n",
     "\n",
-    "This notebook can be used to load and anlysis model learnt from all xgboost bindings, including distributed training. "
+    "This notebook can be used to load and analysis model learnt from all xgboost bindings, including distributed training. "
    ]
   },
   {