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

tutorial/receiving.rst

@@ -149,6 +149,7 @@ Upon running this, you should see:
     BNS = 0.95
     NSBH = 0.01
     BBH = 0.03
+    MassGap = 0.0
     Terrestrial = 0.01
     HasNS = 0.95
     HasRemnant = 0.91