Add docs to clarify naming

The problem

We don't have any documentation of our domain model or naming, which is causing some confusion.

Definition of "done"

• add docs clarifying naming and domain models
• docs in fgen which clarify that we wrap objects, rather than 'calculators'
• docs that clarify that model is thing that holds dy/dt, solver is thing that allows solving the model, stepper is thing which specifies step kind to use when solving

Additional context

First draft happened in !63 (closed)