implement tests for variance calculation of RandomVariables, except for BoxedPDF

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

...ationlevel/functions/util/tests/BoxedPDFVarianceTest.java

...tions/util/tests/ExponentialDistributionVarianceTest.java

...cationlevel/functions/util/tests/LiteralVarianceTest.java

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

.../functions/util/tests/NormalDistributionVarianceTest.java

...pplicationlevel/functions/util/tests/PMFVarianceTest.java