Add Significant field to VO

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The confided field is set to be an integer (to be consistent with the used convection used for the field "HardwareInj" and "OpenAlert"). No clear support for Boolean fields in VO standard.

    <Param dataType="int" name="Significant" ucd="meta.number" value="1">
      <Description>Indicates that this event is a significant alert if 1, no if 0</Description>
    </Param>

The same convention and capitalization of the fields:

    <Param dataType="int" name="HardwareInj" ucd="meta.number" value="0">
      <Description>Indicates that this event is a hardware injection if 1, no if 0</Description>
    </Param>
    <Param dataType="int" name="OpenAlert" ucd="meta.number" value="1">
      <Description>Indicates that this event is an open alert if 1, no if 0</Description>
    </Param>