Comment (by Roland Haas):

There was some old discussion (and two proof of concept implementations) for an external, switchable con2prim thorn (at that point for GRHydro only) http://lists.einsteintoolkit.org/pipermail/users/2017-February/005326.html and demo implementations in the branch: https://bitbucket.org/einsteintoolkit/einsteinevolve/branch/rhaas/con2primng in the Zelmani_Con2PrimNG thorn (method 1) and the ExternalGRHydroCon2Prim thorn (method 2).

Though neither one is going to be quite “right” since they are both tied to GRHydro and a layer that can hook to either IllinoisGRMHD or GRHydro on one side and either the 2018 Siegel et al. or the 2020 Kastaun et al. method my be preferable.

--
Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2414/include-con2prim-framework-by-siegel