Refactor Riemann solvers, add other schemes

[sbryngelson]

1. The Riemann solvers module currently doesn't separate between the physical fluxes (\rho u^2 + p, for example) and the numerical fluxes (e.g., what comes out of HLL/HLLC/Lax-Friedrichs). This is an easy win and an improvement to the code.

2. We can easily add more rudimentary Riemann solvers like Lax-Friedrichs and its sisters (global LF, local LF, Rusanov) and maybe others, like Roe. Some of these were in older archived versions of MFC available in the GH org., though became unwieldy as we added physics. These are useful for some numerics testing, and they become easy to add once 1. is accomplished.