Bug 181121 Model should provide utility method for cube to get correspond dimension key.
In model cube handle, the relation of cube data set and dimension data sets are managed by a series of dimension condition handles.We need model to add utility method so that we can easily get corresponding dimension data set column names according to given cube dimension keys.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/core/MemberRef.java
* 
	 * @param module
	 *            the module
	 * 
	 * @param element
	 *            the element from which to retrieve the structure
	 * @return the value of the referenced structure
	 */
	public Structure getStructure( Module module, DesignElement element )
	{
		if ( propDefn.isList( ) )
		{
			List list = getList( module, element );
			if ( list == null )
				return null;
			
			if ( depth == 1 )
			{
				Structure struct = getStructure( list, 0 );
				if ( struct == null )
					return null;
				if ( member[1] == null )
					return struct;
				
				assert !member[0].isList( );
				return (Structure) struct.getProperty( module, member[0] );
			}
			else if ( depth == 2 )
			{
				return getStructure( list, 1 );