Cast values used as array idxs to int.

The postprocessing script modified here (gstlal_cohfar_plot_stats) is called when analyzing injection runs (there will be an error in the log files that automatically create the report pages), and is useful for assigning FARs when checking whether changes to background collection makes differences to the FAR assignment. The purpose of the script is to plot the FAR contours in the chisq-cohsnr parameter space.

Currently the program fails as several values that are used as array indicies are cast to floats, rather than integers.

These values have now been cast to integers, and the script is useable.

Request Andrew and/or Alex to sanity check before merging. I can provide demonstration of the point of failure.