Split Form Shallow Water Equations

The nonlinear shallow water solvers, in 1-D and 2-D, exhibit spurious flows when topography is spatially variable.

[Gassner, Winters, & Kopriva (2016)](https://www.diva-portal.org/smash/get/diva2:1315753/FULLTEXT01.pdf) and [Wintermeyer et al. (2017)](https://www.sciencedirect.com/science/article/pii/S0021999117302310) have shown that a discretization based on the split form of the shallow water equations is well-balanced (preserves lake-at-rest) and is entropy conservative, provided an appropriate Riemann flux is given. Additionally, they've shown that minimal modifications are needed to adjust a conservative form solver to obtain an equivalent implementation.