Revisit the name 'Generator'

[marcoct]

This conflicts with the usage in https://en.wikipedia.org/wiki/Generator_(computer_programming). This confusion actually visibly arose once during the summer school.

[fsaad]

Also coincides with a previous use of "generator" in bayeslite (now known as an analysis schema), which users, even those familiar with probabilistic modeling and inference, found as a confusing notion:

https://github.com/probcomp/bayeslite/blob/1566fd91bd74e81d497d715c1a6fd44b23926424/src/metamodel.py#L118