Release v0.9

This issue collects breaking changes we want to include in version v0.9: - [ ] Change `default_example` to use https://github.com/NumericalMathematics/DispersiveShallowWater.jl/blob/6881e80061c553c5a3f1ec45888376186f48b8ca/examples/serre_green_naghdi_1d/serre_green_naghdi_soliton.jl instead, see discussion in #256 - [ ] Introduce argument `plot_analytical` in plot recipes, which takes the role of the current `plot_initial` and let `plot_initial` indeed plot the initial condition (at time $t_0$), see discussion in #254