Update experimental warnings before release

[patrickersing]

Before making a first release we need to update the experimental warnings.
Especially with the new features, that are about to be introduced with the multilayer equations in #30, we have to discuss which functions need to be experimental.

[andrewwinters5000]

This is particularly relevant toward the upcoming extension of the positivity limiter to the multi-layer equations. Abstracting the fluxes and water heights to enable easy iteration over nlayers should make the syntax of applying the limiter much easier compared to the current naming and storage conventions used in ShallowWaterTwoLayerEquations1D and ShallowWaterTwoLayerEquations2D.