Update to the Riemann solver unit tests

See merge request !529

SSE support

See merge request !519

Missing factor of \pi in the argument of the Green function for the top-level mesh potential calculation.

Improvements to GIZMO implementation

See merge request swift/swiftsim!524

Make the brute-force tests use the correct cosmological parameters and also call the chemistry functions.

Removed unused isothermal Riemann solver. Added generic input and output debug checks for all remaining Riemann solvers. Made sure HLLC Riemann solver works for 0 input pressure. Made sure all quantities are set to vacuum if density/mass or pressure/energy is zero.