implement additional pmf and boxedPdf tests

...applicationlevel/functions/util/RandomVariableHelper.java

...plicationlevel/functions/util/tests/BoxedPDFMeanTest.java

...mm/applicationlevel/functions/util/tests/PMFMeanTest.java

...icationlevel/testfiles/boxed-pdf-zero-intervals.functions

....applicationlevel/testfiles/double-pmf-negative.functions

....mm.applicationlevel/testfiles/double-pmf-unary.functions

...ationlevel/testfiles/double-pmf-uniform-distrib.functions

....mm.applicationlevel/testfiles/int-pmf-negative.functions

...dml.mm.applicationlevel/testfiles/int-pmf-unary.functions

...licationlevel/testfiles/int-pmf-uniform-distrib.functions