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/PropertyStructure.java
* 
	 * @see org.eclipse.birt.report.model.core.Structure#getIntrinsicProperty(java.lang.String)
	 */
	protected Object getIntrinsicProperty( String propName )
	{
		assert false;
		return null;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.model.core.Structure#setIntrinsicProperty(java.lang.String,
	 *      java.lang.Object)
	 */
	protected void setIntrinsicProperty( String propName, Object value )
	{
		assert false;
	}
	/**
	 * Makes a copy of this property structure map.
	 * 
	 * @return IStructure of this property.
	 * @throws CloneNotSupportedException
	 * 
	 */
	public Object clone( ) throws CloneNotSupportedException
	{
		PropertyStructure clone = (PropertyStructure) super.clone( );
		clone.propValues = new HashMap( );
		for ( Iterator iter = propValues.keySet( ).iterator( ); iter.hasNext( ); )