Introduce additional abstraction layer for the SWE/SWEWD to reduce code duplication

[patrickersing]

We currently have to duplicate some functionality, that is already implemented in Trixi.jl, such as for example the FluxHLL.
As discussed here it could be a good idea to introduce an additional abstraction layer to minimize the code duplication e.g. a function

@inline function (numflux::FluxHLL)(u_ll, u_rr, orientation_or_normal_direction,
                                    equations::AbstractShallowWaterEquations{NDIMS})

that works for both the SWE in Trixi.jl and the SWEWD in TrixiShallowWater.jl.

However, when introducing this we need to be careful that these functions work for all sub-types.