Roadmap KdV eq.

The following I want to do next regarding the KdV eq. - [x] Add KdV to Benchmark results, see #208 - [x] add nondim2prim + prim2nondim functions to be able to compare results from $$u_t + u u_x + u_{xxx}=0$$ PDE to the current implementation $$\eta_t+\sqrt{g D} \eta_x+3 / 2 \sqrt{g / D} \eta \eta_x+1 / 6 \sqrt{g D} D^2 \eta_{x x x} = 0$$, see #217 - [ ] more docs - [ ] split up the ode - make it possible to use IMEX in DSW.jl, see also #165 - [ ] add more conversion_functions, right now only waterheight is working - [ ] possibly add more examples - [ ] more unit tests