reorganize compressible riemann solver interface to prepare for SphericalPolar geometry

[zhichen3]

1. Split up riemann_cgf to 2 parts: getting the conserved states then flux construction
2. added an overall riemann_flux interface to find the flux directly if there is no need to get the conserved states.
3. make consFlux general for both multi-zone U_state or single zone U_state.

[zingale]

I think that this should still be named CGF, since this is the Colella, Glaz, Fergoson solver. We will add more solvers and want to keep them straight.

Then you new wrapper can be used to get the flux as needed by calling the appropriate solver.

[zhichen3]

now riemann_flux is the general interface to directly get flux from riemann solvers given direction. And riemann_cgf now gives conserved states.

[zingale]

this is very nice.