Add `ParameterSet.implicit_forcing` method

For the usecase where forcing is already included in the (example) parameterset, it is not strictly necessary to also create an `ewatercycle.forcing.Forcing` object.

However, it might still be useful to have this object, even if it's just for inspection. Perhaps we could add a method (not sure how to name it, maybe something like `implicit_forcing`) to automatically generate the 'default' forcing object that comes with the parameterset.