Skymap comparison tests - for all five events

This adds reference data and tests for skymaps for representative datasets from all 5 events highlighted in the review as deserving of them.

Technically, this is done with a new run_skymap.sh script that takes the reference input file and makes a skymap from it. skymap_comparison takes an event parameter that controls which event input is used, and there's a new function, get_skymap_comparison_events that lists all valid events. For completeness, there are also two new parameters, save_prob and save_cohsnr to skymap_comparison that allows you to save the probability map and the coherent SNR map, respectively.

For backwards compatibility, event is an optional parameter (and defaults to the old event); and the run_gw170817_skymap.sh still exists (but now just calls run_skymap.sh with default parameters).