New skymap plots

New plots as used for the online alert system. It uses ligo.skymap package that has then been added to extras. Minor fixes have been done to two other functions that were using a deprecated signature of pandas.DataFrame.set_axis() since Pandas==2.0. The two new functions are skymap_hpx_new and skymap_alert_new. They are meant to replace skymap_hpx and skymap_alert but as the signatures are different, independent functions are created for now.