Add deprecation warning for domain-object inputs to ratinterp().
Add deprecation warning for domain-object inputs to ratinterp().

We don't need to use a domain class in ratinterp() to solve dispatching
problems anymore, so it would be nice if we could get rid of it as a
supported syntax in the future.

ilovejs/chebfun,alshedivat/chebfun,asgeirbirkis/chebfun