Merge branch 'gizmo_hllc_solver_fix' into 'master'
Fixed bug in HLLC Riemann solver flux calculation. This fixes #216 (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. See merge request !257