WIP draft of randomness concept

[colinleach]

I started this because I thought porting it from Python would be a quick win. Spectacularly wrong! It took more than a week, and still isn't great.

To be fair, the way relevant functionality is spread around Julia packages is kind of a mess and needs improvement.

Pushing .gitignore was a mistake, but it shouldn't matter. It just leaves out any Jetbrains configuration.

I'll try to get back to more central concepts soon, though I may also submit a small Statistics concept. That overlaps with the random stuff, so I made notes on both in parallel.