Generate performance test data

[johnny-morrice]

We need to decide the cases for which the system is to be optimized. It's hard to do without data!

As a default, I'd like to discover the shapes of data which are most naturally performant given the system architecture, and then optimize for that case.

So I want to be able to generate test data with respect to statistical parameters.

We will then choose the sets we're most interested in, and so optimize.