[Reviewer Matt] More fixes to remove dependency on latest and greatest GCC

Matt

Can you review please.  There are two changes here - test improvements to the binomial distribution UT case, and the change to remove the dependency on std::exponential_distribution.

On the former, Alex was seeing intermittent failures of this test case on his build system, even though I could run it multiple times on the main repo server.  I'm speculating that the difference is because the test case does not randomly seed the random number generator.  Indeed, changing it to seed the generator allowed me to reproduce the failures on the main repo server.  I've therefore relaxed the test slightly (now checking the results are within 0.05 of the expected distribution rather than 0.01).  This did not fail in 10,000 repeats of the test using a different seed on each test.

On the latter, I haven't added a test case yet as it uses a simple well known formula to do the transform from a uniform distributed variable, and testing is much harder for a continuous distribution.  I'll continue to look at this and add one if it seems feasible.  I have run the code through the UTs.

Mike