Point-implicit method for friction computation
The explicit Euler method will not work if bottom friction is included as source term. Therefore an implicit method/treatment is needed for the friction source term (e.g. see Franz Simons Thesis "A robust high-resolution hydrodynamic numerical model for surface water flow and transport processes within a flexible software framework", https://doi.org/10.14279/depositonce-9589 in Chapter 3.5).