Properly include RapidPE P_astro

Currently there are a few issues when creating circulars (specifically update for now) when including RapidPE. These include

• Citation is not included when displaying p_astro numbers
• Wording is confusing for p_astro, where it is stated that we are assuming the event is astrophysical

Here's a example update example for S230823a:

SUBJECT: LIGO/Virgo/KAGRA S230823a: Updated Source Classification

The LIGO Scientific Collaboration, the Virgo Collaboration, and the KAGRA Collaboration report:

We have conducted further analysis of the LIGO Hanford Observatory (H1), LIGO Livingston Observatory (L1), and Virgo Observatory (V1) data around the time of the compact binary merger (CBC) candidate S230823a (GCN Circular ***CITE ORIGINAL GCN ID, e.g. 25012***).

Assuming the candidate is astrophysical, the updated parameter estimation based classification of the GW signal, in order of descending probability, is Terrestrial (72%), BBH (27%), NSBH (1%), or BNS (<1%).

For further information about analysis methodology and the contents of this alert, refer to the LIGO/Virgo/KAGRA Public Alerts User Guide https://emfollow.docs.ligo.org/userguide/.