Refactor p_astro igwn alert consumer

This PR refactors the IGWNAlertConsumer classes and relates to #11 (closed).

Note that for some reason I had some issues using the click library when running the igwn_alert.client object. I couldn't diagnose what the issue is, but for some reason using any other CLI tool (python-fire, argparse, or just running it with hard-coded variables) had no issues. For this reason the consumer scripts are written with argparse and the coinc.xml upload script just uses click as per normal.