GRHD: Make parameters of cons2prim's velocity filter configurable

This adds the following parameter to the GRHD section:

• c2p_threshold_velocity_limiter: Threshold y₀ = ρhW² below which the velocity is limited by a filtered division following Dumbser+ 2024.

I found emperically that this value strongly influences the reflection and generation of perturbations in the TOV star tests. In particular, using 1e-4 gives a PSD of the central density evolution in which all peak frequencies are wrong compared to literature (e.g. Three-dimensional numerical general relativistic hydrodynamics. II. Long-term dynamics of single relativistic stars, Font+, 2002, table 1). Using instead a value of 1e-6 gives better results. However, this seems to come at the expense of stability, because with lower limiter values we appear to more easily apply surface heating, e.g. epsilon or tau display artifacts at the surface.

The default value of c2p_threshold_velocity_limiter is now set to 1e-6 because of the above observation. This also explains why updated the ref test in this PR.