HLL-MSF solver

The code implements an HLL-type well-balanced numerical scheme that accurately captures internal hydraulic jump in two layer shallow water equations

MSF stands for Moving and Stationary Shockwave Fixes.

the method was introduced in an article avalibale in: https://doi.org/10.2166/hydro.2023.312

for more about HLL-MSF solver see also: https://doi.org/10.1002/fld.5076