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
* within the top-level list.
	 * <p>
	 * property.list[n].member
	 * 
	 * @param prop
	 *            the property definition
	 * @param n
	 *            the list index
	 * @param memberDef
	 *            the definition of the member
	 */
	MemberRef( ElementPropertyDefn prop, int n, StructPropertyDefn memberDef )
	{
		propDefn = prop;
		assert propDefn != null;
		assert propDefn.getTypeCode( ) == IPropertyType.STRUCT_TYPE;
		assert propDefn.isList( );
		assert n >= 0;
		assert memberDef != null;
		refType = PROPERTY_LISTn_MEMBER;
		index[0] = n;
		member[0] = memberDef;
		depth = 1;
	}
	/**
	 * Reference to the nth item in a first or second level list.
	 * <p>
	 * property.member.list[n] <br>
	 * 
	 * property.list[n] <br>