Added p_astro_gstlal to gwcelery tasks

• p_astro computation is done for gstlal triggers. The machinery is present in ligo.p_astro package (https://git.ligo.org/lscsoft/p-astro)

• values of p_astro run by hand on replay GW170817 can be seen in log messages here (https://gracedb-playground.ligo.org/events/G2933/view/)

Things to be done

• Add retry options to compute_p_astro task.
• Adding compute_p_astro to the event/superevent canvas workflow
• Unittesting tasks in p_astro_gstlal
• Reduce the size of the ranking_data_G322589.xml.gz test file to a few hundred kilobytes or less. Might involve reducing histogram resolution.