Fixed the handling of 'defaultChild' missing value strategy for leaf nodes. HT: Alistair Sutherland
Renamed local variables

The value of the target field should be called "targetValue" for greater
consistency with the code example given in the Javadoc of interface
org.jpmml.evaluator.Evaluator.
Cleaned up code
Improved the handling of 'returnLastPrediction' no true child strategy
Simplified control structures