Improved code coverage
Extracted methods TreeModelEvaluator#handleTrue and #handleDefaultChild
Improved validation criteria
Allow comparison between boolean and numeric types

A boolean field is almost always of categorical operational type. The
PMML specification prohibits categorical fields from being used in
comparison operations (ie. "lessThan", "lessOrEqual", "greaterOrEqual",
"greaterThan"). Nevertheless, popular PMML producer software are known
to generate SimplePredicate elements where the boolean field is treated
identically to numeric fields (eg. "<SimplePredicate
field="boolean_flag" operation="lessThan" value="0.5">").

This change permits a boolean field to be compared against a numeric
field. The implementation is not symmetric. That is, an attempt to
compare a numeric field against a boolean field will cause an exception
to be thrown.