Refactor run_models

[pschultzendorff]

Currently, different logic and bookkeeping regarding time stepping, logging, progressbars, and solvers is crammed into run_time_dependent_model. This makes it hard to see the time stepping and solver logic.

Analog to the rewrite of NewtonSolver in #1448, we want to introduce a base class ModelRunner, with childs StationaryModelRunner, TimeDependentModelRunner, and IterativeModelRunner that clearly separate the logic into different methods.

[pschultzendorff]

An initial attempt is pushed on the working branch. Separating into several runner classes for time_dependent model, static_model, (and iterative_model) may be too much. We should discuss this.