Bug 213126 Add support for DataEngine-leve dataset cache
In many cases the DataEngine-leve dataset cache is useful which ensure that a dataset is executed one time during rendering one report. To support this feature a new dataset property should be added. The Birt customer can choose whether this dataset need to be cached or not by setting the property in deisgn time.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/DataSetHandle.java
assert propHandle != null;
		return propHandle.iterator( );
	}
	/**
	 * Returns an iterator over filter. The iterator returns instances of
	 * <code>FilterConditionHandle</code> that represents filter condition
	 * object.
	 * 
	 * @return iterator over filters.
	 * 
	 * @see org.eclipse.birt.report.model.api.elements.structures.FilterCondition
	 */
	public Iterator filtersIterator( )
	{
		PropertyHandle propHandle = getPropertyHandle( IDataSetModel.FILTER_PROP );
		assert propHandle != null;
		return propHandle.iterator( );
	}
	/**
	 * Returns the code of the beforeOpen method. This is the script called just
	 * before opening this data set.
	 * 
	 * @return the code of the method
	 */
	public String getBeforeOpen( )
	{
		return getStringProperty( ISimpleDataSetModel.BEFORE_OPEN_METHOD );
	}
	/**
	 * Sets the code for the beforeOpen method. This is the script called just