Reword the p_astro statement to improve terseness, use an
apporpriate number of digits, and keep it near the closely
related em_bright paragraph.

Note that I added a macro to render very high probabilities as
">99%" and very low probabilities as "<1%", which is now used
in both the p_astro and the em_bright paragraphs.

The text was:

    The categorical astrophysical probabilities for the candidate
    event as computed by the P_astro pipeline are:
     BNS probability: 0.8593197490253501
     BBH probability: 0.0
     NSBH probability: 0.0
     Terrestrial probability: 0.1406802509746499

The text is now:

    The classification of the signal, in order of descending confidence,
    is BNS (78%), terrestrial (22%), BBH (<1%), or NSBH (<1%).