PUB-147: Unified handling of NAs in generated code.

Now each branch decision contains additional comparision with
Double.NA since we need to put all NA into the left branch and
cannot depend on comparision NA < val, which for finite val returns false.
Titanic GBM test relating to PUB-147.

The objective of the test is to verify that generated model
and interpretted model handle NAs in the same way.
Simplified rescaling exp-logic in GBM score method (PUB-144)