The name `arb::model` did not clearly describe the role of the class, while `arb::simulation` better captures that this is an instantiation of a model for the purpose of running a simulation, as distinct from the description of a model represented by an `arb::recipe` instance.

* Rename sources `model.{hpp,cpp}` to `simulation.{hpp,cpp}`.
* Rename class `arb::model` to `arb::simulation`.
* Update docs and tests to suit.