SRHD: Update cons2prim (!149)

This PR replaces the SRHD's own c2p with that of GRHD.

The following parameters are added to the SRHD section:
- `id_smooth = false`: If `true` initial data is smoothed with a Bernstein filter.
- `av_regularization = "none"`: Controls which AV regularization is to be used.
- `c2p_cold_eos_parameters = [ 100.0, 2.0 ]`: Parameters of cold polytropic EoS for c2p, analogous to GRHD.
- `av_drag = 0.0`: AV dragging factor to control how rapid AV can change, analogous to GRHD and EulerEq.
- `av_recompute_substeps = false`: If `true` AV recomputation is triggered on every substep in the evolution.