Use traits for composable generator

[pat-alt]

Maybe an idea could be to use traits: https://ahsmart.com/pub/holy-traits-design-patterns-and-best-practice-book/.

Instead of using a single base struct that can be functionally composed in different ways, we could use traits to define the behavior of different generators. This would still be (perhaps even more) user-friendly, but potentially more flexible and cleeaner.

Also address:

• Add convergence_conditions field to generators #156
• Metadata for generators #256