OPAL-2190: Many improvements to contingency table.
OPAL-2190: CrossableVariableOracle and CategoricalVariableOracle are now extending TableVariableSuggestOracle
OPAL-2190: Cleaning up VariableNature that is no longer needed because the nature is indexed. Double-quoting datasource and table name.
Merge pull request #411 from emorency/OPAL-2190-crosstab

OPAL-2190: Contingency table improvements