Changed CQL to XCQL conversion, so that element for relation value in

Changed CQL to XCQL conversion, so that element for relation value in
modifier is 'comparison' rather than 'relation' so that it conforms to
 http://www.loc.gov/standards/sru/xml-files/xcql.xsd
When a comparison value is omitted, the st.relation in cql_node will
be NULL instead of "=". This has the effect that comparison will be
omitted in those cases.