Fixed bug in HLLC Riemann solver flux calculation.

Merged Bert Vandenbroucke requested to merge gizmo_hllc_solver_fix into master

This fixes #216 (closed) (again, but now all cases).

I basically overlooked a tiny detail in the real GIZMO and changed the order of some operations that cannot be interchanged. Square test is now verified to work with all 3 possible Riemann solvers.

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