Initialization of the hyperbolic approximation parameters of SGN

https://github.com/NumericalMathematics/DispersiveShallowWater.jl/blob/b4cba0b29eb4569703ed96c5e2f53d5508432adb/src/equations/hyperbolic_serre_green_naghdi_1d.jl#L142-L144 For variable bathymetry, we should use $$w = \dot \eta + \frac{3}{2} \dot b = -h \nabla \cdot v + \frac{3}{2} v \cdot \nabla b$$ In the current implementation, the variable bathymetry part is missing.