GRHD: Refactor c2p (!150)

- Mainly refactors `c2p` in order to allow to ignore the total energy density in the recovery procedure.
This is possible for cold equations of state, where we don't need to evolve tau.

- The `c2p` now also incorporates a conservative fixing that we adapted from from arXiv:1306.4953, Appendix.

- We had to update reftest data for the SRHD simple wave and GRHD bondi tests.
Especially the GRHD bondi tests improved from not using tau in the reconstruction.

- We also updated the default value `atm_treshold_factor` so that it is now slightly large than one.
This fixes some errors where we misidentified atmosphere states as being below atmosphere, before the c2p procedure started.

- Because this PR was cherry-picked from the fa/doublecartoon+av branch, we also brought over the changes in the naming of variables of the doublecartoon formulation (which should have went into another PR, but I was too lazy to entangle those changes first).

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I noticed that the number of allocs in `rhs` sections are now quite increased.
This is particularly bad for the SRHD tests. My guess is this is due to this refactor now passing on both EoSs as parameters to the c2p routine.