Allow for runtime control of rhs TAG

[SamuelCupp]

Since the pairing of RHS variables with their evolution variables is now done via a tag, it would be helpful for this to be controllable at runtime.

For an example use case, the Avec variables in HydroBaseX should be usable as the evolution variables directly instead of requiring the data to be copied into a new variable first. This requires that the rhs tag be controllable via a parameter, and that the thorns using this tag read it properly at runtime based on the parameter. The basic idea of this is already implemented (see e.g. prolongation_type in the original HydroBase thorn), but I don't know if this mechanism is fully supported by the code that uses this tag.

At the least, this would require adding a runtime parameter for controlling the rhs variable for the Avec groups, as well as the ability to turn it off if A isn't being evolved. The option to turn it off is almost certainly not implemented in the current rhs infrastructure, so this capability would need to be added.