Firedrake integration for the poisson problem

Dear Andreas,
sometimes ago you commented on one of my firedrake issues, regarding coupling firedrake with some BEM library.
I would like to try to replace (extend) our current library with `pytentials`. Up to now, I managed to install `meshmode` as well as `pytentials` within our firedrake docker container.

The original plan was to use the code from the meshmode examples `to_firedrake.py` and `from_firedrake.py`. Before I start implementing I just wanted to make sure that this is still up-to-date, or if there is already some interface using the UFL external operators as reported within your publication.

best wishes and thanks for your help
Florian