Add MassGap classification

Example field for mass-gap added in the preliminary, initial and update alert xml files. gwcelery!359 (comment 77026)

CC: @deep.chatterjee @shasvath.kapadia

_static/MS181101ab-1-Preliminary.xml

@@ -65,6 +65,9 @@ xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml
             <Param name="BBH" dataType="float" value="0.03" ucd="stat.probability">
                 <Description>Probability that the source is a binary black hole merger</Description>
             </Param>
+            <Param name="MassGap" dataType="float" value="0.0" ucd="stat.probability">
+                <Description>Probability that the source has at least one object between 3.0 and 5.0 solar masses</Description>
+            </Param>
             <Param name="Terrestrial" dataType="float" value="0.01" ucd="stat.probability">
                 <Description>Probability that the source is terrestrial (i.e., a background noise fluctuation or a glitch)</Description>
             </Param>
@@ -74,7 +77,7 @@ xsi:schemaLocation="http://www.ivoa.net/xml/VOEvent/v2.0 http://www.ivoa.net/xml
                 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">
-                <Description>Probability that at least one object in the binary has a mass that is less than 3 solar masses</Description>
+                <Description>Probability that at least one object in the binary has a mass that is less than 2.83 solar masses</Description>
             </Param>
             <Param name="HasRemnant" dataType="float" value="0.91" ucd="stat.probability">
                 <Description>Probability that a nonzero mass was ejected outside the central remnant object</Description>