Matplotlib use 'agg' backend

After talking to Carl, I realized I can avoid this by not doing ssh -X when I ssh in to the machine. Anyway, I think the real fix is to add matplotlib.use("Agg") here https://git.ligo.org/lscsoft/parallel_bilby/-/blob/master/parallel_bilby/analysis.py#L19 and here: https://git.ligo.org/lscsoft/parallel_bilby/-/blob/master/parallel_bilby/ptemcee_analysis.py#L17 (edited)