Compute p_astro for MDC events
It will be handy to have the p_astro file generated for MDC events so that we can include it in sample events for astronomers.
Note that the values in the coinc_event.likelihood and sngl_inspiral.Gamma1 columns from the "First Two Years" coinc.xml file didn't look right compared to what's in the p_astro unit test data. I assume that gstlal's uses of these columns have changed. Please check the assumptions that I made in that regard.