implement additional exponential and normal distribution tests and simple boxed pdf tests

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

...icationlevel/functions/util/tests/MeanCalculatorTest.java

...plicationlevel/testfiles/boxed-pdf-four-samples.functions

...l.mm.applicationlevel/testfiles/boxed-pdf-unary.functions

...ationlevel/testfiles/expo-distrib-negative-rate.functions

...plicationlevel/testfiles/expo-distrib-zero-rate.functions

...ionlevel/testfiles/normal-distrib-negative-mean.functions