Bug 161174 Expect to set/get interval range with a 'String' param in GroupHandle.
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/GroupHandle.java
* <li><code>INTERVAL_WEEK</code>
	 * <li><code>INTERVAL_DAY</code>
	 * <li><code>INTERVAL_HOUR</code>
	 * <li><code>INTERVAL_MINUTE</code>
	 * <li><code>INTERVAL_SECOND</code>
	 * <li><code>INTERVAL_INTERVAL</code>
	 * 
	 * </ul>
	 * 
	 * @return the interval value as a string
	 */
	public String getInterval( )
	{
		return getStringProperty( GroupElement.INTERVAL_PROP );
	}
	/**
	 * Returns the interval of this group. The input value is defined in
	 * <code>DesignChoiceConstants</code> and can be one of:
	 * 
	 * <ul>
	 * <li><code>INTERVAL_NONE</code>
	 * <li><code>INTERVAL_PREFIX</code>
	 * <li><code>INTERVAL_YEAR</code>
	 * <li><code>INTERVAL_QUARTER</code>
	 * <li><code>INTERVAL_MONTH</code>
	 * <li><code>INTERVAL_WEEK</code>
	 * <li><code>INTERVAL_DAY</code>
	 * <li><code>INTERVAL_HOUR</code>
	 * <li><code>INTERVAL_MINUTE</code>
	 * <li><code>INTERVAL_SECOND</code>