Attached is a patch adding Richardson extrapolation routines, tests,
documentation, and an example.  Please let me know if I can answer any
questions.

> The closest routines are in sum/ but that's not an ideal name so we
> can just create a new extrap/ directory for it.

The routines sit in gsl_extrap.h and build under extrap/.  I included
the documentation under sum.texi as part of the Series Acceleration
chapter.  The technique's math fits there more closely than anywhere
else.

- Rhys