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/Structure.java
* Gets the value of the member. If the value has not been set, the default
	 * value for the member will be returned.
	 * 
	 * @param module
	 *            the module
	 * @param memberName
	 *            the member name
	 * @return the member is defined on the structure and the value is set,
	 *         otherwise null.
	 */
	public final Object getProperty( Module module, String memberName )
	{
		PropertyDefn prop = (PropertyDefn) getDefn( ).getMember( memberName );
		if ( prop == null )
			return null;
		return getProperty( module, prop );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.core.IPropertySet#getProperty(org.eclipse.birt.report.model.elements.ReportDesign,
	 *      org.eclipse.birt.report.model.metadata.PropertyDefn)
	 */
	public Object getProperty( Module module, PropertyDefn propDefn )
	{
		assert propDefn != null;
		Object value = getLocalProperty( module, propDefn );
		if ( value == null )
			return propDefn.getDefault( );
		if ( value instanceof ElementRefValue )