problems including integral-based symbolic operations (e.g. mean, integral) in boundary operators

exterior-stokes-err.pystokes.py

The script exterior-stokes-err.py should show the weird behavior when using integral-related (but not layer potential integrals) terms in a boundary operator for solving.

The script stokes.py contains the wrapper classes for stokeslet/stresslet use. (I changed exterior-stokes-err so that it's expecting stokes to be in the same directory.)

Note that this requires the changes to the stresslet in sumpy/kernel.py for which I submitted a merge request.