[MONDRIAN-1587] Drillthrough SQL is invalid on hsqldb.
SQL formerly included redundant parentheses in cases where drillthrough was generating a single field IN list, for example "field" IN ( ('item1'), ('item2' ).  Eliminated the parentheses except in multivalue IN lists.
Validated changes against postgres, greenplum, mysql, oracle, sqlserver and hsqldb.
Merge pull request #85 from mkambol/mondrian-1587

[MONDRIAN-1587] Drillthrough SQL is invalid on hsqldb.