refactor into multiple test files; implement additional tests

...evel/functions/util/tests/AbstractMeanCalculatorTest.java

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

...functions/util/tests/ExponentialDistributionMeanTest.java

...pplicationlevel/functions/util/tests/LiteralMeanTest.java

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

...evel/functions/util/tests/NormalDistributionMeanTest.java

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

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

...applicationlevel/testfiles/normal-distrib-float.functions