Implement `source_term_manning_friction` for the `ShallowWaterEquations`

[patrickersing]

It all looks good to me! Just one small thing for the sake of consistency; in the Monai tutorial we could maybe change source_terms_manning_friction to be source_term_manning_friction on lines 265 and 319 in the file docs/src/tutorials/elixir_shallowwater_monai_tsunami.jl.

@andrewwinters5000 Right, we should make that consistent. However, we currently have not implemented source_term_manning_friction for the standard ShallowWaterEquations1D, so I think we would need to extend the existing TrixiSW function inside the tutorial. I don't think that is optimal for an introductory example.

In order to fix the inconsistent naming of the friction source term in the tutorial elixir_shallowwater_monai_tsunami.jl, we should implement source_term_manning_friction for the ShallowWaterEquations and use it in the tutorial.

Originally posted by @patrickersing in #128 (comment)