I think I agree with that, while it's not as complete as having a whole `InterpParams` class I think it is likely to be safe and is definitely less unwieldy.  However, if we do use this kludge we ought to document it here, in `Interpolant.h` and at the Python layer too.  Otherwise I'm not fundamentally objecting to the `InterpParams` class, I just think that it might be overkill now and so we should document a working solution and come back to this later if need be.