Implement Rusanov MHD solver

Using the functions implemented in experimental/MHDSystemExperimental.hpp, we can now use a generic solver to do simple MHD simulations. The Rusanov solver is already implemented for hydro in src/ars/HydroRiemannRusanov.hpp

A few new MHD tests (initial conditions) should also be implemented (maybe these shock tubes https://homepages.dias.ie/jmackey/jmac/node7.html ?)

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