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/data.cc

@@ -77,12 +77,12 @@ inline bool MetaTryLoadGroup(const std::string& fname,
 
 // try to load weight information from file, if exists
 inline bool MetaTryLoadFloatInfo(const std::string& fname,
-                                 std::vector<float>* data) {
+                                 std::vector<bst_float>* data) {
   std::unique_ptr<dmlc::Stream> fi(dmlc::Stream::Create(fname.c_str(), "r", true));
   if (fi.get() == nullptr) return false;
   dmlc::istream is(fi.get());
   data->clear();
-  float value;
+  bst_float value;
   while (is >> value) {
     data->push_back(value);
   }