Feature request: ability to turn off evolution for variables at runtime

[SamuelCupp]

In Carpet, I could control which variables would actually evolve based on what I registered with MoL. This means that for e.g. IllinoisGRMHD I could select to only evolve the extra entropy or tabulated EOS variables if I'm actually using them.

In CarpetX, I am not aware of how to do this. If I have ent_star and ent_star_rhs in the interface.ccl and set the tags, the code fails at CycleTimeLevels due to the variable being invalid. I can easily fix this by adding a function that just writes 0 to everything at the same place the RHS are calculated. However, a preferred option would be to somehow control whether the variable should be evolved. Alternatively, if STORAGE is ever made to not be always on, then hopefully it would include logic to handle this since entropy evolution variables are only given storage if (evolve_entropy).