Improve the consistency of predicates in "If-then-else" expressions

The predicate or conditional-expression in the evaluation of the If-expression allows literal numeral values to be part of the predicate. In the parser, if it sees a literal value of 1 then it would be true otherwise it is false; however it is entirely plausible to extend that definition to cover all values > 0 to be true otherwise values < 0 will be interpreted as false.

E.g. this should print No! but instead it prints "Yes!"

scala> val s = dml(""" if(-42) print("Yes!") else print("No!") """)

scala> ml.execute(s)

Yes!

SystemML Statistics:

Total execution time: 0.012 sec.

Number of executed Spark inst: 0.

res0: org.apache.sysml.api.mlcontext.MLResults = None