Implementing a small for from Carl Haster to fix cbcBayesMCMC2pos.py
@vivien this implements a fix from Carl Haster. lalinferencemcmc runs were crashing for me when calling cbcBayesMCMC2pos.py, due to importing bayespputils.py and then requiring the ability to open a window. Using the 'Agg' matplotlib backend fixes this.