Bug fix -- filterwidth() wasn't properly enforcing that it needs derivs.
Also remove type checking table entry for aastep, which was never implemented.
Implement aastep.

Change docs to remove description of the "filter" argument to aastep,
which I don't think is really necessary and I don't think was used in
previous renderers I worked on that provided that functionality.  It
makes the implementation a lot simpler.  I can always add it later if
requested.