Port chebfun.chebpade() over from Chebfun v4.
Port chebfun.chebpade() over from Chebfun v4.

This has only been copied over and reformatted a bit and, in particular,
has not yet been tested.

alshedivat/chebfun,asgeirbirkis/chebfun,ilovejs/chebfun