Implement RANDOM_UNIFORM function

[travisfranck]

Vensim's RANDOM_UNIFORM (along with other RANDOM_XYZ functions) have not been implemented yet. SDE should support the most used functions, like RANDOM_UNIFORM, RANDOM_POISSON, and RANDOM_NORMAL. Because they all require a random number generator, the suggestion is to focus on one, RANDOM_UNIFORM, to implement the foundation.

Vensim's RANDOM documentation, including the algorithm choice, is documented here:
https://www.vensim.com/documentation/noise_rng.html

A link to some code for the algorithm is here:
https://github.com/MersenneTwister-Lab/SFMT

I asked Vensim forums about tips and pitfalls. TomF provided a good reply on this thread here
https://www.ventanasystems.co.uk/forum/viewtopic.php?p=32192

The highlight of the thread is that:

Equation sorting is deterministic in Vensim, but I'm not sure we could readily tell you exactly what the rules are.
Another source of variation is likely to be calls during initialization - anything that causes one more or one less will break symmetry.

[travisfranck]

Additional thoughts:

• Someone suggested that reproducing Vensim's random number system is "a fool's errand". They suggested having an exogenous stream of random numbers that drives both Vensim and SDE. That would take the algorithm out of the equation, but maybe not the equation ordering.

Some options for implementation:

1. Try the above exogenous stream of random numbers (external file)
2. Try to Vensim's algorithm
3. Just use C's native rand() function
4. Find a way to capture Vensim's random numbers and save it to a file, which is then read into SDE