Dear GalSim users -
For those who want to include flexion in their weak lensing image simulations, I would like to point you towards a package developed by Justin Bird, called GalFlex:
It is a python package that can easily interface with GalSim; see the GalFlex demos for an example of how to do this (as well as examples using GalFlex on its own).
We may explore a more direct integration of GalFlex into GalSim in future, but this is still TBD.