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
* property. <strong>member </strong>.member <br>
	 * property.member. <strong>list[n] </strong> <br>
	 * property.member. <strong>list[n] </strong>.member <br>
	 * 
	 * property. <strong>list[n] </strong>[.member] <br>
	 * property.list[n].member. <strong>list[n] </strong>[.member] <br>
	 * property.list[n]. <strong>member </strong>.member
	 * 
	 * @return the definition of the structure pointed to by the reference
	 */
	public IStructureDefn getStructDefn( )
	{
		switch ( refType )
		{
			case PROPERTY :
			case PROPERTY_LISTn :
			case PROPERTY_MEMBER :
				return propDefn.getStructDefn( );
			case PROPERTY_LISTn_MEMBER :
			case PROPERTY_LISTn_MEMBER_MEMBER :
			case PROPERTY_LISTn_MEMBER_LISTn :
			case PROPERTY_LISTn_MEMBER_LISTn_MEMBER :
			case PROPERTY_MEMBER_MEMBER :
			case PROPERTY_MEMBER_LISTn_MEMBER :
			case PROPERTY_MEMBER_LISTn :
				return member[0].getStructDefn( );
			default :
				assert false;
				return null;
		}