RichFutureKernel: Use known functions in histogram tests

Christopher Rob Jonesrequested to merge
RichFutureKernel-Use-Known-Distributions-For-Hist-Tests into master

Update the test to use known functional forms, with apriori known statistical quantities, to test calculations from Gaudi.

See gaudi/Gaudi#351 (closed) for more details.

Edited by Christopher Rob Jones

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