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
memberName );
		assert memberDefn != null;
		refType = PROPERTY_MEMBER;
		member[0] = (StructPropertyDefn) memberDefn;
		index[0] = -1;
		depth = 1;
	}
	/**
	 * Reference to a member of a top-level structure.
	 * <p>
	 * property.member
	 * 
	 * @param prop
	 *            the element property definition which is structure or
	 *            structure list type
	 * @param memberDef
	 *            the structure member definition of the element property
	 *            definition
	 */
	MemberRef( ElementPropertyDefn prop, StructPropertyDefn memberDef )
	{
		propDefn = prop;
		assert propDefn != null;
		assert propDefn.getTypeCode( ) == IPropertyType.STRUCT_TYPE;
		assert !propDefn.isList( );
		refType = PROPERTY_MEMBER;
		member[0] = memberDef;
		index[0] = -1;
		depth = 1;
	}