Rearrange p_astro and em_bright fields to match formatting in user guide

They should be gathered into Groups, their names should be adjusted, and the descriptions should be updated.

        <Group type="Classification">
            <Description>
                Source classification: binary neutron star (BNS), neutron star-black hole (NSBH), binary black hole (BBH), or terrestrial (noise)
            </Description>
            <Param name="BNS" dataType="float" value="0.95" ucd="stat.probability" unit="">
                <Description>Probability that the source is a binary neutron star merger</Description>
            </Param>
            <Param name="NSBH" dataType="float" value="0.01" ucd="stat.probability" unit="">
                <Description>Probability that the source is a neutron star - black hole merger</Description>
            </Param>
            <Param name="BBH" dataType="float" value="0.03" ucd="stat.probability" unit="">
                <Description>Probability that the source is a binary black hole merger</Description>
            </Param>
            <Param name="Terrestrial" dataType="float" value="0.01" ucd="stat.probability" unit="">
                <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
            </Param>
        </Group>
        <Group type="Properties">
            <Description>
                Qualitative properties of the source, conditioned on the assumption that the signal is an astrophysical compact binary merger
            </Description>
            <Param name="HasNS" dataType="float" value="0.95" ucd="stat.probability" unit="">
                <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
            </Param>
            <Param name="HasRemnant" dataType="float" value="0.91" ucd="stat.probability" unit="">
                <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>
            </Param>
        </Group>