`GRHD`: Implement reflection bdry conditions for `valencia1d, spherical1d, rescaled_spherical1d` formulations

It works nicely with spherical1d, rescaled_spherical1d, but fails miserably with valencia1d. Not sure what the problem there is, because for both rescaled_spherical1d and valencia1d I need to interpolate onto the origin after unscaling the conservatives, because the volume factor comes with an extra r that makes the conservatives vanish there. However, for valencia1d and spherical1d I also have singular source terms, which I don't have in rescaled_spherical1d.