Test randomization functions

[JamesPHoughton]

Test the following Vensim functions:

• EXPRND
• LOGNORMAL
• RANDOM NORMAL
• POISSON
• RANDOM UNIFORM

We may have to be clever about how we do it. Maybe taking sums of large numers of samples with different parameters?

[JamesPHoughton]

The caching issue is complex here, as it doesn't depend on the stock. The value of the call to rand should probably not be cached, as that would introduce artificial covariance. But the functions that contain the call should absolutely be cached, so that they have the same value for any of their downstreams...

Should write a variable that uses the rand, and is called in two different elements, and then make sure they are the same, which would not be the case if the full tree was called each time...