[Users] Make refluxing available to all

Hi all,

sorry for the spam, but I would like to give a little bit of advertisement to this ticket:

https://trac.einsteintoolkit.org/ticket/1862

I propose a patch for the Refluxing thorn that will decouple it from GRHydro. With the proposed changes, any evolution thorn will be able to register variables for refluxing in a very simple way (see the GRHydro_Refluxing thorn in the ticket for an example). For instance, it should be possible to add conservative AMR to the IllinoisGRMHD code. My code "WhiskyTHC" is already making use of this capability.

I would be happy for any feedback on these patches and I hope they can make their way into the Einstein Toolkit.

Thank you and best,

David