Re-name solver to stepper

The problem

The euler forward and RK4 methods are really methods about how you step a model forward. They do not specify everything about how to solve a model (e.g. they know nothing about inputs or outputs) hence we should rename these (and their base class) from solver to stepper.

Definition of "done"

• #51 (closed)
• re-named solver to stepper for these classes
• split out the actual solving stuff (e.g. solve_ivp) and stepping stuff into different folders

Additional context

First draft happened in !63 (closed)