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
/**
	 * Reference to the the named member in the nth structure in the top-level
	 * list.
	 * <p>
	 * property.list[n].member
	 * 
	 * @param prop
	 *            the property definition
	 * @param n
	 *            the list index
	 * @param memberName
	 *            the name of a member
	 */
	MemberRef( ElementPropertyDefn prop, int n, String memberName )
	{
		propDefn = prop;
		assert propDefn != null;
		assert propDefn.getTypeCode( ) == IPropertyType.STRUCT_TYPE;
		assert propDefn.isList( );
		assert n >= 0;
		StructPropertyDefn memberDefn = (StructPropertyDefn) propDefn
				.getStructDefn( ).getMember( memberName );
		assert memberDefn != null;
		refType = PROPERTY_LISTn_MEMBER;
		index[0] = n;
		depth = 1;
		member[0] = memberDefn;
	}
	/**
	 * Reference to the identified member of the structure at the nth position